More than 55% of world population today has its roots in monotheistic beliefs. For if there are many gods, then the one who is most powerful of them all should be the one true God, the God of Gods whose will over-rides all others.  If all are equal in power but have opposing conscious, then there cannot be creation and system but only chaos.  However if all are entirely equal to one another in authority and power and intention, then they are surely one.

Being muslims, our belief in Tauheed are more strict and in addition to evidence of creation, cause & effect, take their roots in the belief in the unseen;

Those who believe in the unseen and keep up prayer and spend out of what We
have given them.
Quran [2:3]

God is surely Unseen. But Prophet Muhammad (SAW), a man famous for his honesty and trustworthiness for 40 years, a man who risked his life for 13 years fighting the chieftains of Mecca without any mass following, refusing all worldly bounties, a man (SAW) whose family fought and sacrificed their sons for his (SAW) message at Karbala, this man, Prophet Muhammad (SAW), is not the unseen, his message is not the unseen. In Iqbal's words, one can deny God, but how will one deny Muhammad (SAW)?

Being Muslims, our utmost emphasis is on Tauheed or oneness of Allah. We believe, that without this belief, nothing else will matter.

Surely Allah does not forgive that anything should be associated with Him, and
forgives what is besides that to whomsoever He pleases; and whoever associates anything with Allah, he devises indeed a great sin.
Quran [4:48]

Allah has the power over all things, and he will forgive who ever he wishes. Who has the authority to question Allah in his decisions? But Allah has promised in his book that he may forgive anything other than shirk.

Infact, Allah has promised forgiveness to ghair muslims who believe in Tauheed in its true letter and spirit;

Surely those who believe, and those who are Jews, and the Christians, and the
Sabians, whoever believes in Allah and the Last day and does good, they shall have their reward from their Lord, and there is no fear for them, nor shall they grieve.
Quran [2:62]

Tauheed forms the basic building block of Islam and embodies itself in the Kalimah. Prophets throughout the history of mankind have reminded us of the same message where as we as humanbeings continued to corrupt the message on account of our natural battle with our nafs.

The companions of Musa (AS) watched all of the mircles of Musa (AS) with their own eyes, and being delivered from the Pharoah after lengthy tribulation, reverted back from Tauheed within just 40 days of Musa's (AS) absence;

And Musa's people made of their ornaments a calf after him, a (mere) body,
which gave a mooing sound. What! could they not see that it did not speak to them nor guide them m the way? They took it (for worship) and they were unjust. And when they repented and saw that they had gone astray, they said: If our Lord show not mercy to us and forgive us we shall certainly be of the losers. And when Musa returned to his people, wrathful (and) in violent grief, he said: Evil is it that you have done after me; did you turn away from the bidding of your Lord? And he threw down the tablets and seized his brother by the head, dragging him towards him. He said: Son of my mother! surely the people reckoned me weak and had well-nigh slain me, therefore make not the enemies to rejoice over me and count me not among the unjust people.
Quran [7:148-150]

It requires no more discussion being muslims, how without Tauheed we have wasted our creation. But it requires discussion as to what Tauheed means and why in today's advanced world and age of science, it matters, given that it is a primitive concept and discussion.

To understand Tauheed, it is important to understand two feuds which will have unanimous interpretation amongst the masses. I have selected these two feuds because; one has shaped humanity, and the other has shaped Islam as a religion. One is at an astronomical scale and the other one is as powerful as a myth. One has its roots with the first Prophet, Adam (AS), and the other has its roots with the last prophet, Muhammad (SAW). There are atleast three things common in these feuds. Both are about revolting against an authority based on their set of rationale, in both the feuds one side has been cursed for eternity, and most importantly, in both these feuds, all the parties involved are firm in Tauheed, the dispute is not tauheed.

The first is Iblis' revolt against Allah.

Iblis was a monotheist, he was a believer in the Tauheed of Allah. He had worshipped Allah with angels. Who could have understood the Tauheed of Allah? In Quran's words, Iblis was never and still is not a mushrik.

Like the Shaitan when he says to man: Disbelieve, but when he disbelieves, he
says: I am surely clear of you; surely I fear Allah, the Lord of the worlds.
Quran [59:16]

He said: My Lord! then respite me till the time when they are raised.
Quran [15:36]

The lord of Satan is Allah. Iblis testified that he fears Allah and that Allah is the lord of the worlds.

The revolt of Iblis was not on the Tauheed of Allah;

He said: O Iblis! what excuse have you that you are not with those who make
obeisance? He said: I am not such that I should make obeisance to a mortal whom Thou hast created of the essence of black mud fashioned in shape. He said: Then get out of it, for surely you are driven away: And surely on you is curse until the day of judgment.
Quran [15:32-35]

And the vengence of Iblis was not limited to Adam (AS);

He said: Tell me, is this he whom Thou hast honored above me? If Thou shouldst
respite me to the day of resurrection, I will most certainly cause his progeny to perish
except a few.
Quran [17:62]

The biggest laeen in our text is a believer in the Tauheed of Allah. He believes that Allah is one, lord of the worlds, is to be feared. However, Iblis is laeen because he did not understand the authority of Allah, the authority that Tauheed demands, the authority Allah bestowed on Adam (hence prostration by all angels). Iblis is laeen because he did not limit his enmity with Adam, to Adam (AS), but vowed to take the fight to the progeny of Adam (AS).

With the above summary in mind, the striking resemblance with the second feud.

The second is Imam Hussain's battle at Kerbala.

The opposition of Imam Hussain (AS) consisted of muslims, Namazis, Hajis, consisted of sons of companions of Prophet (SAW).

Umar bin saad offered the morning prayers on Ashura and then released an arrow in the direction of the tent of Imam Hussain (AS) saying "Bear witness that I was the first to strike".
Abridged and taken from Maqtal al Husayn (AS) by Abu Mikhnaf

Umar bin Saad (laeen), the captain of the Yazeedi forces in Kerbala was the son of Saad bin Abi Waqas (one of the ten blessed companions of Ashra Mubashra). The governer of Kufa, the comander in chief of opposition of Imam Hussain (AS), Ubaidallah bin Ziyad (laeen), was the claimed grandson of Abu Sufyan, the uncle of Prophet (SAW). Yazeed (laeen) himself, was the nephew of Prophet (SAW) as his aunt, Umm Habiba (RA) was married to Prophet (SAW).

These people were not mushriks. These people believed in one Allah, recited the Kalimah of Grandfather of Imam Hussain (AS) while mutilating his very body on the 10th of Muharram.

These people did not limit their enmity to Hussain (AS) or his father Ali (AS) but extended their enmity to the progeny of Banu Hashim, the progeny of Prophet Muhammd (SAW) himself.

When Imam Ali bin Husain (a.s.) entered the court of Yazid along with the members of his family they were brought near the blessed head of Imam Husain (a.s.) which was placed in a tray and Yazid (l.a.) was striking it with a stick and was saying,

Bani Hashim played a game to acquire rulership.There was neither any message which descended nor any revelation. How I wish my elders who were killed in Badr would be able to see this and the injury stricken people of Khazraj who were severely wounded in the Battle of Ohud would see then they would be pleased with me and would praise me and say, O Yazid, may your hands never be paralysed! We have taken a heinous revenge from their family and through this revenge of mine – I have avenged my ancestors of Badr. Had I not done this then I did not belong to the progeny of Khindif, and I not taken revenge from the family of Ahmed (Holy Prophet (s.a.w.a.)) for what they did.
Shaikh al-Tabrisi writes in his book, Al-Ehtejaaj, vol. 2, p. 307

Janab Zainab (AS) bint Ali (AS) in one of her famous sermons reminded Yazid of his ancestral loyalty and his origins to the enmity to Islam;

This is not surprising and there is no expectation of mercy or compassion from the children of those who have loved to chew the liver of the noble martyrs of Islam. The one whose flesh has been developed by drinking the blood of good and noble people, who bears enmity with the Chief of the Prophets (s.a.w.a.), gathers and increases the hatred of people against him (s.a.w.a.), raised a sword against the radiant face of Holy Prophet (s.a.w.a.), one who is the severest opponent of Allah among all the Arabs and the biggest denier of His Prophet (s.a.w.a.), who has openly declared his enmity against them and is the biggest disbeliever and rebel in the sight of Allah the Most High.
Sermon of Janab Zainab (AS) in the courtyard of Yazid

These people became laeen because they fought against the prince of paradise, they did not realize the authority of Imam Hussain (AS), an authority bestowed by Allah himself.

Imam Hussain (AS) continued the sermon by praising and glorifying Allah and sending blessings upon Muhammad (SAW) and God's angels and prophets:
"Trace my descent and reflect upon who I am. Then look towards yoruself and be self critical. Do you think it lawful to kill me and violate my sanctity? Am i not the son of the daughter of your prophet (SAW). The son of the trustee of the Prophet (SAW) and his cousin, the first one to believe in Allah and embrace what his messenger brought from his lord? Is Hamzah, lord of the martyrs not my uncle? Is Jafar who flies with wings in heaven not my uncle? Do you not testify to the famous statement of the Prophet (SAW) about me and my brother Hassan that we are the lords of the youths of paradise? If you believe that what I am saying is the truth and it is indeed the truth, for by Allah I have never told a lie since i realized that Allah hates liars and that it harms only those who engage in falsification. If you do not believe me than ask those are are among you they will tell you about me. They will confirm that the heard this statement from Rasool Allah (SAW). Is this not sufficient to stop you from shedding my blood? If you doubt this prophetic statement do you also doubt that I am the son of the daughter of your messenger? By Allah there is no such person but me, search among yourselves or the entire world. Tell me whether you seek revenge for someone among you whom I have killed for property that i usurped or for an injury that I inflicted?
Abridged and taken from Maqtal al Husayn (AS) by Abu Mikhnaf

There can be many take aways from these two feuds which were not with mushrikeens, but their roots very much lie in Tauheed.

Tauheed is to believe in the authority of Allah. It is very important to understand that although we all testify that authority belongs to Allah, we do not understand it. Absolute Authority cannot be questioned, justified or rationalized. When we start questioning authority, and try to question its logic, righteousness etc, that means that we think that authority needs to make sense and needs to abide by certain rules. Once the authority has to abide by certain rules, it does not remain "The" authority any more. Allah created because he can. Simply put. Can someone do anything about it? Absolute authority belongs to Allah and no one can question him.

The absolute authority belongs to no one but Allah.

Who created the heavens and the earth and what is between them in six periods,
and He is firmly established on the throne of authority; the Beneficent Allah, so ask
respecting it one aware.
Quran [25:59]

It is absolute because no one gave him that authority, it was his, it is his essence. All the other authorities that are out there, are given by Allah, as much to any one as he wills.

Allah gave the authority to Adam;

And when your Lord said to the angels, I am going to place in the earth a khalif,
they said: What! wilt Thou place in it such as shall make mischief in it and shed blood,
and we celebrate Thy praise and extol Thy holiness? He said: Surely I know what you do
not know.
Quran [2:30]

Disregarding this authority, even if you believe in Allah's authority, is as if disregarding Allah's authority because it is Allah himself who distributed this authority. Who are we to question who Allah gives authority and who he does not.

Musa (AS) was given authority by Allah;

And certainly We sent Musa with Our communications and a clear authority,
Quran [11:96]

The argument of Quran against false gods and idols is not that they are idols, but that they have not been given the authority by Allah;

He said: Indeed uncleanness and wrath from your Lord have lighted upon you;
what! do you dispute with me about names which you and your fathers have given? Allah
has not sent any authority for them; wait then, I too with you will be of those who wait.
Quran [7:71]  

And how should I fear what you have set up (with Him), while you do not fear that
you have set up with Allah that for which He has not sent down to you any authority;which then of the two parties is surer of security, if you know?
Quran [6:81]

And they serve besides Allah that for which He has not sent any authority, and
that of which they have no knowledge; and for the unjust there shall be no helper.
Quran [22:71]

It is clear that Tauheed does not mean to believe in the authority of Allah, but also in the authority of those who have been given as such by Allah himself.

O you who believe! obey Allah and obey the Messenger and those in authority
from among you; then if you quarrel about anything, refer it to Allah and the Messenger,
if you believe in Allah and the last day; this is better and very good in the end.
Quran [4:59]

To kill is as if all mankind has been killed, but there is no sin at all if Allah has given that authority to some one to kill someone.

And do not kill any one whom Allah has forbidden, except for a just cause, and
whoever is slain unjustly, We have indeed given to his heir authority, so let him not
exceed the just limits in slaying; surely he is aided.
Quran [17:33]

To commit adultery is a punishable crime by Shariah law, because the man and woman have no authority over each other to commit such an act, without nikah. After nikah, the same act authorizes both and gives each other rights over each other. The same act with the authority of Allah becomes Mustahib, something which was haram without the authority of Allah.

Hur's (AS) namaz in the camp of Yazid (laeen) without understanding the authority of Imam Hussain (AS) is one thing, the same act of namaz, in the camp of Hussain (AS) becomes another.

What if Ali's (AS) wilayat is indeed authorized by Allah through Prophet (SAW) at Gadeer? If it announcement of Mawlayat of Ali (AS) is something else, then given Ali's (AS) stature, there is no harm in following him an opposing his enemies. After all Sunnis do believe in following the ways of the companions as well. But what if the authority of Imamat is a reality? 

What authority did the Prophet (SAW) leave his Ahlulbayt (AS) behind with?

Yazid b. Hayyan reported, I went along with Husain b. Sabra and 'Umar b. Muslim to Zaid b. Arqam and, as we sat by his side, Husain said to him: Zaid. you have been able to acquire a great virtue that you saw Allah's Messenger (may peace be upon him) listened to his talk, fought by his side in (different) battles, offered prayer behind me. Zaid, you have in fact earned a great virtue. Zaid, narrate to us what you heard from Allah's Messenger (may peace be upon him). He said: I have grown old and have almost spent my age and I have forgotten some of the things which I remembered in connection with Allah's Messenger (may peace be upon him), so accept whatever I narrate to you, and which I do not narrate do not compel me to do that. He then said: One day Allah's Messenger (may peace be upon him) stood up to deliver sermon at a watering place known as Khumm situated between Mecca and Medina. He praised Allah, extolled Him and delivered the sermon and. exhorted (us) and said: Now to our purpose. O people, I am a human being. I am about to receive a messenger (the angel of death) from my Lord and I, in response to Allah's call, (would bid good-bye to you), but I am leaving among you two weighty things: the one being the Book of Allah in which there is right guidance and light, so hold fast to the Book of Allah and adhere to it. He exhorted (us) (to hold fast) to the Book of Allah and then said: The second are the members of my household I remind you (of your duties) to the members of my family. He (Husain) said to Zaid: Who are the members of his household? Aren't his wives the members of his family? Thereupon he said: His wives are the members of his family (but here) the members of his family are those for whom acceptance of Zakat is forbidden. And he said: Who are they? Thereupon he said: 'Ali and the offspring of 'Ali, 'Aqil and the offspring of 'Aqil and the offspring of Ja'far and the offspring of 'Abbas. Husain said: These are those for whom the acceptance of Zakat is forbidden. Zaid said: Yes.
Sahih Muslim Book 031, Number 5920

Yazid b. Hayyan reported: We went to him (Zaid b. Arqam) and said to him. You have found goodness (for you had the honour) to live in the company of Allah's Messenger (may peace be upon him) and offered prayer behind him, and the rest of the hadith is the same but with this variation of wording that lie said: Behold, for I am leaving amongst you two weighty things, one of which is the Book of Allah, the Exalted and Glorious, and that is the rope of Allah. He who holds it fast would be on right guidance and he who abandons it would be in error, and in this (hadith) these words are also found: We said: Who are amongst the members of the household? Aren't the wives (of the Holy Prophet) included amongst the members of his house hold? Thereupon he said: No, by Allah, a woman lives with a man (as his wife) for a certain period; he then divorces her and she goes back to her parents and to her people; the members of his household include his ownself and his kith and kin (who are related to him by blood) and for him the acceptance of Zakat is prohibited.
Sahih muslim Book 031, Number 5923

We have not understood the absolute authority of Allah, for if we had, we would not have judged people on the amount of Salat they performed but the orphans they fed. We would not have killed and judged people to hell on account of their beliefs. We would have understood, that Allah has given our parents authority over us to be unconditionally respected and be followed (until contrary to Islam).

How can you believe in Tauheed of Allah when what you measure is not fair, you do not remember death and you do not respect the authority that Allah himself has given over you, to the prophet, to the imam, to your teacher, parents, to your elders, to those who are better than you and more gifted, to those who are more learned and pious than you?

The Tauheed of Allah is more complicated than the simplicity of his unity. It is in essence, his absolute authority to which all of us must submit to willingly, for the day when we will all be called out unwillingly, we will have no authority even over our bodies and our own flesh will testify against us.