Were all the wives of the Messenger of Allah, peace and blessings be upon him, Muslims before marriage, or did one of them embrace Islam at his hand after she had been a disbeliever?

All the wives of the Prophet, peace and blessings be upon him, were Muslim believers. Allah Almighty chose them for him and made them Mothers of the Believers. Safiyyah bint Huyayy embraced Islam before the Prophet, peace and blessings be upon him, married her, as evidenced by the Prophet's statement to Aisha when she described Safiyyah as Jewish: "Do not say that, for she embraced Islam and her Islam was good." If the Prophet had married a Jewish woman, the Companions would not have differed concerning the permissibility of marrying a woman from the People of the Book. As for Khadijah, may Allah be pleased with her, the Prophet married her before the prophethood, and she was the first to believe in him after the revelation.