What is the ruling on one who believed in the prophethood of Messenger Muhammad (peace be upon him) and did not deny it, but died upon the religion of the People of the Book (such as the Torah)? Is he considered a Muslim or a disbeliever?

Mere knowledge of the truthfulness of the prophethood of Prophet Muhammad (peace be upon him), without affirming it and following his religion, is not considered a salvific faith. Everyone who adheres to a religion other than Islam is considered a disbeliever and will abide eternally in Hellfire. Allah the Almighty has clarified that whoever seeks a religion other than Islam, it will never be accepted from him. Ibn Taymiyyah mentioned that mere knowledge and reporting about prophethood is not faith unless it is accompanied by commitment and submission. An example of this is the case of Abu Talib, who knew of the Prophet's truthfulness but refrained from affirming it out of love for the religion of his forefathers.