In order to be a Muslim, that is, to surrender oneself to God, it is necessary to believe in the oneness of God,in the sense of His being the only Creator, Preserver,Nourisher, etc. But this belief, later called Tawhid Ar-Rububiyyah, is not enough.Many of the idolators knew and believed that only the Supreme God could do all this. But this was not enough to make them Muslims.
To tawhid ar-rububiyyah, one must add tawhid al-‘uluhiyyah. That is, one acknowledges the fact that it is God alone who deserves to be worshiped, and thus abstains from worshiping any other thing or being.Having achieved this knowledge of the one true God, man should constantly have faith in Him, and should allow nothing to induce him to deny truth.
When faith enters a person’s heart, it causes certain mental states that result in certain actions. Taken together, these mental states and actions are the proof for the true faith. The Prophet said:“ Faith is that which resides firmly in the heart and which is proved by deeds”.