TO BELIEVE IN ALLAAH AND HIS MESSENGER ﷺ
Among the obligations of the heart is to believe in Allaah and what Allaah revealed, and to believe in the Messenger of Allaah ﷺ and what he conveyed.
Explanation: Every accountable person1 has to have the correct belief in Allaah. This means for one to believe firmly in one’s heart that Allaah is One without any partner, nothing resembles Allaah, Allaah is the Creator of everything, and Allaah is the One Who controls the entire creation. Nothing occurs unless Allaah Willed for it to occur and nothing exists unless Allaah brings it into existence, i.e. creates it. This is the best and most important obligation — to believe properly in Allaah.
Just as it is an obligation to believe in Allaah, one has to believe in what Allaah revealed. This means that one has to believe in what Allaah revealed to Prophet Muhammad ﷺ. In believing in what was revealed to Prophet Muhammad ﷺ, one must have the firm belief in their heart that all of what Allaah revealed to Prophet Muhammad ﷺ is true. One cannot object to that and one must completely submit to the truthfulness of the religion of Islaam.
Every accountable person also has to have the correct belief in Prophet Muhammad ﷺ. This means that one must revere Prophet Muhammad ﷺ in the proper way without attributing to him things that do not befit him and without giving him a status which is higher than his status ﷺ.
One has to believe in what the Prophet ﷺ conveyed from Allaah. So, if someone says, “I believe in Allaah and I believe in Prophet Muhammad ﷺ but I do not believe in the truthfulness of such-and-such rule with which the Prophet ﷺ conveyed”, then this person is not a believer. He is not a Muslim, even if he claims to be a Muslim, because if one claims that the Prophet ﷺ conveyed even one thing which was not truthful, then he is belying the Prophet ﷺ. The one who belies the Prophet ﷺ is not a true believer in the Prophet ﷺ; such a person is a disbeliever.
1This is the pubescent, sane person, who received the call of Islaam. If someone is pubescent, sane, and received the call of Islaam, there is no difference in opinion among the Muslim scholars that this person has to believe properly in Allaah and the Messenger of Allaah ﷺ and embrace the religion of Islaam.