Muslims, to believe in the prophethood and mission of Muhammad ﷺ; if they refused, they were neither believers nor Muslims. For by denying him they had, in effect, rejected a prophet sent by Allah Most High and spurned a revelation He Himself had brought down.

Belief in Muhammad ﷺ is a prerequisite for believing in the prophethood of all the prophets:

Thus, affirming the prophethood of Muhammad ﷺ is the very condition for affirming the prophethood of every other messenger. It is impossible truly to believe in any prophet while repudiating Muhammad, the Messenger of Allah ﷺ; whoever rejects his prophethood is even more disposed to reject the prophethood of the others.

This is clarified in several ways:

First: The earlier prophets foretold his coming and commanded their nations to believe in him. Whoever denies his prophethood thereby accuses those prophets of falsehood in what they reported and opposes them in what they enjoined regarding faith in him. Affirming him is therefore inseparable from affirming them; if the corollary is negated, the principal is necessarily negated as well. The proofs of this inseparability will follow immediately, and together they yield certainty that he ﷺ was mentioned in the divine scriptures on the tongues of the prophets.

Second: The call of Muhammad ﷺ is identical to the call of all the messengers before him, from the first to the last. Hence, whoever rejects his call rejects the call of all his prophetic brethren, and such rejection constitutes disbelief. Accordingly, believing in his call ﷺ and following him is obligatory.

Third: The signs and evidences demonstrating the soundness of his prophethood and his truthfulness ﷺ are many times greater than the signs granted to the messengers before him (1). There is no prophet who possessed a miracle compelling faith in him except that Muhammad ﷺ possessed its like.