Matters will not be correct and sound except with the presence of tawheed.
*In the Name of Allaah, the Extremely Merciful, the Bestower of Mercy…
Shaykh Saalih al-Fawzaan:
…This is what is obligatory for the du`aat (callers) to precede upon in their call, that they should focus upon warning against shirk and calling to tawheed before everything, otherwise their call will not be upon the manhaj (methodology) of the Messenger (sallallaahu `alayhi wa sallam).
Allaah sent the Messenger (sallallaahu `alayhi wa sallam) with warning against shirk and calling to tawheed, so it is essential to lay this foundation down first of all. After that, the person can direct his attention to the rest of the matters, since matters will not be correct and sound except with the presence of tawheed.
If all of the people were to abandon zinaa (fornication), intoxicants, stealing, and they took on every virtuous quality from deeds and behavior, but did not abandon shirk, there would be no benefit in these matters and it will not benefit them. Whereas, if the people were to remain free of shirk, but had major sins less than shirk, then there is hope for such a person that Allaah will either forgive him or punish him in accordance with the level of his sins. But his final destination will be to Paradise because he is a person of tawheed.
Tawheed is the fundamental basis and foundation. There is no salvation except with the presence of tawheed firstly. Therefore, it is obligatory to concentrate and always give attention to it and to call the people to it and to teach it to the people and to explain it to them, what is the meaning of tawheed and what is the meaning of shirk. The Muslim must be aware of this matter and attain certainty concerning it. He should check himself so that he does not fall into anything from shirk or infringe tawheed. This matter is essential and the da`wah must precede upon this basis.(1)
(1) Sharh Usool-ith-Thalaathah, Shaykh Saalih al-Fawzaan. Translated by Aboo Talhah Dawud Burbank -rahimahullaah.