What is the ruling on the marriage of a Muslim woman who has doubts about the existence of Allah and the truthfulness of Muhammad to a Muslim man?

Doubt concerning the existence of the Creator is of two types: The first is a satanic whispering (waswasah) accompanied by a firm belief, and this should be repelled by seeking refuge [in Allah] and by not delving into it. The second is when doubt settles in the heart, leading to a statement or an action, and this is disbelief (kufr).

If the doubt is a mere whispering, then it is permissible for a man to marry. However, if the doubt has settled, then marriage is not permissible until after repentance. The existence of the universe and humanity is proof of the Creator; it is impossible for anything to exist without a creator.

As for the truthfulness of the message, it is indicated by: the Prophet's—peace and blessings be upon him—virtuous character before his prophethood, the miracles with which Allah supported him, especially the Noble Qur'an, his foretelling of future events that came to pass, and his triumph over his enemies and the conquest of lands.