Praise be to Allah, Lord of the worlds, Who promised the righteous people to ultimately prove successful. May peace and blessings be upon the master of the early and late generations, Prophet Muhammad (peace be upon him) and upon all prophets, messengers, and righteous people. It is admittedly true that Tawhid (belief in the Oneness of Allah/monotheism) and belief in Allah's Messengers is the most important and greatest obligation. Similarly, attaining this kind of knowledge is the best and most honorable science. Moreover, there is a pressing need for explaining it in detail. It is for these reasons that I take upon myself to clarify this issue in brief. Indeed, this great subject deserves to be given greater consideration. So, I pray to Allah (May He be Glorified and Exalted) to grant us all success in words and deeds and to protect us all from mistakes and errors.
Undoubtedly, Tawhid is the first and foremost obligation. It is the first call made by all messengers (peace be upon them) and is the core of Da`wah (calling to Islam). In the EverGlorious Qur'an, Allah (May He be Glorified) says about all messengers: And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): "Worship Allâh (Alone), and avoid (or keep away from) Tâghût (all false deities i.e. do not worship Tâghût besides Allâh)."
Allah (May He be Glorified and Exalted) points out that He sent to each Ummah (nation based on one creed) a messenger preaching: worship Allâh (Alone), and avoid Taghut (false gods). This is the call of all messengers to their respective Ummahs.
This call means to believe in the Oneness of Allah, for the dispute occurred between the messengers and their Ummahs only originated in respect of the Oneness of `Ibadah (worship). Almost all Ummahs acknowledge that Allah is the Lord, the Creator, and the Provider. They also know many of His Names and Attributes. However, the dispute and enmity, from the time of Nuh (Noah peace be upon him) to the present day rises concerning the issue of dedicating `Ibadah to Allah Alone. Messengers were sent to call people to dedicate `Ibadah to Allah, believe in His Oneness, and abandon dedicating `Ibadah to anyone other than Him. The opponents held the belief that they should worship Him along with others as well, not to devote `Ibadah to Him Alone.
Tawhid [of worship] i.e. to worship Allah Alone.