Allah Most High said in the year of the Farewell Pilgrimage:
“Today I have perfected your religion for you and completed My favor upon you.”
Furthermore, even after the Quran had been revealed and the religion perfected, if only part of the religion reached a person, he was required to believe in everything the Messenger brought in general, and in whatever had reached him from the Messenger in detail. As for what had not reached him and which he had no means of knowing, he was required to know it in detail only when it reached him.
Likewise, if a person believed firmly in the Messenger and died before the time for prayer began, or before any deeds became obligatory upon him, he died with complete faith in all that was required of him. Once the time for prayer begins, however, he must pray: something has now become obligatory upon him that was not obligatory before. Similarly, someone capable of performing the pilgrimage and engaging in jihad is required to have detailed belief and act accordingly in ways not required of someone else.
Thus, the requirements of faith vary according to the stage reached in the descent of revelation from heaven and according to whether or not its message has reached the legally accountable individual. These are among the factors that cause assent itself to vary. Its character also differs according to ability, inability, and other factors that give rise to obligation; these likewise cause the deeds required to differ. Clearly, what is obligatory upon each of these individuals is not identical to what is obligatory upon another.
If, then, the very content of faith required under a single revealed law varies and admits of degrees—even though all these forms share a common core, such as acknowledging the Creator, devoting religion sincerely to Him alone, and acknowledging His messengers and the Last Day in general—it is clear that when someone fulfills some of what is obligatory upon him but not the rest, his fulfillment of faith is partial, just as other obligations may be partially fulfilled.