Making the Religion purely for Allaah is the foundation of the Religion. And it is its axis around which its mill revolves. And it is the Tawheed that Allaah sent His messengers with and for which He revealed His books. It is that which the prophets called to and for which cause they made Jihaad. They commanded it and encouraged it. Allaah says: "Surely, the Religion (i.e. worship of Allaah) is for Allaah alone." [Surah Zumar: 3]
"And they were commanded not, except that they should worship Allaah, making the Religion (worship) purely for Him alone, and to establish the prayer and give Zakaah." [Surah Al-Bayyinah: 5]
And He says: "Say (O Muhammad): "It is Allaah whom I worship, making my Religion (worship) purely for Him (alone). So worship what you like besides Him." [Surah Az-Zumar 14-15]
And: "Say (O Muhammad): "Verily, my prayer, my sacrifice, my living, and my dying are (all) for Allaah, the Lord of the worlds. He has no partner. And of this I have been commanded, and I am the first of the Muslims." [Surah An’aam: 162-163]
Therefore, this Tawheed is like the foundation of a building. Ibn Al-Qayyim (ra) said: "Whoever wants a lofty building, then let him strengthen, consolidate and put great concern in its foundation. For indeed a lofty building depends on the firmness and precision of its foundation. Thus, (righteous) deeds and levels are the building and Eemaan is its foundation.
So the knowledgeable person strives to rectify and perfect its foundation while the ignorant person constructs his building without any foundation. And consequently it will not be long before his building falls. Allaah says: 'Is then he, who laid the foundation of his building on piety to Allaah and His Good Pleasure, better, or he who laid the foundation of his building on an undetermined brink of a precipice ready to crumble down, so that it crumbled to pieces with him into the Fire of Hell.'" [Surah At-Tawbah: 109] [See the book Al-Fawaaid: pg. 204]
This ayah was revealed concerning the hypocrites who built a masjid to pray in. But when they performed this great and beneficial deed, while their hearts were void of any sincerity (to Allaah), their action did not benefit them in the least. Rather it collapsed and crashed down on them in the Hellfire as is stated in the ayah. Then he (Ibn Qayyim) said: "This foundation consists of two things:
1. Having correct knowledge of Allaah, His Command, His names and Attributes.
2. Devoting oneself solely to Allaah and His messenger and no one else. So this is the firmest foundation that the slave of Allaah can establish his building on. And according to that, he will be able to raise his building however high he wants. So perfect the foundation, uphold the stability and preserve the sanctuary." [See Al-Fawaaid: pg. 204]
Since Tawheed is like the foundation of a building, it is the first command that the people come across when opening the mushaf (to read the Qur'aan) from its beginning. And that command is found in Allaah’s saying: "O mankind! Worship your Lord (Allaah) who created you and those before you so that you may become Al-Muttaqoon (the pious)." [Surah Al-Baqarah: 21]
Then immediately after this, there is a prohibition of what goes against Tawheed, and that is shirk. Allaah says: "Then do not set up rivals unto Allaah (in worship) while you know (that He Alone has the right to be worshipped)." [Surah Al-Baqarah: 22]
This is a splendid point for Allaah did not only command us to worship Him, but He also prohibited us from what nullifies that, which is worshipping others beside Him. So look throughout the book of your Lord and you will constantly encounter it, such as Allaah’s saying: "And verily, We have sent amongst every nation a Messenger (proclaiming): ‘Worship Allaah (alone) and avoid the Taaghoot (all false deities)." [Surah An-Nahl: 36]
And there is also Allaah’s saying: "Worship Allaah and join none with Him in worship." [Surah Nisaa: 36] Shirk is the first of the forbidden matters that Allaah prohibited, as is stated in His saying: "Say (O Muhammad): ‘Come, I will recite what your Lord has prohibited you from: Join not anything in worship with Him.’" [Surah An’aam: 151]
And it is the first advice that Luqmaan gave to his son when he said: "O my son! Join not in worship others with Allaah. Verily, joining others in worship with Allaah is indeed a great wrong." [Surah Luqmaan: 13]
People’s main concerns vary and they are only known for real at the time of death, when the living one counsels the dead one. So among them is he who counsels his wife concerning his wealth, and among them is he who counsels his friend concerning his position and his authority, and among them is he who counsels his son concerning his brothers.
But the best of them is he who counsels the servant of Allaah concerning his Lord. Because of this, Tawheed was the advice and counsel of the prophets at the times of their death. Allaah says: "And this (religion of Islaam) was counseled by Ibraaheem upon his sons and by Ya’qoob, (saying): ‘O my sons! Allaah has chosen for you the (true) religion, so do not die except while being Muslims Or were you witnesses when death approached Ya’qoob? When he said to his sons: ‘What will you worship after me?’ They said: ‘We shall worship your God, the God of your fathers, Ibraaheem, Ismaa’eel, Ishaaq, One God, and to Him we submit (in Islaam).’" [Surah Al-Baqarah: 132-133]
This is why those who call to Tawheed are the best and most honorable of callers. This is because calling (da’wah) to Tawheed is in fact a call to the highest level of Eemaan. Abu Hurairah RAA reported that the Prophet SAWS said: "Eemaan has more than seventy - or more than sixty - branches. The highest of them is (saying): ‘There is no deity worthy of worship except Allaah’ and the lowest of them is to remove a harmful object from the road. And modesty is part of Eemaan."