What should a Muslim man married to a Christian woman do if she converts to Islam and then apostatizes—should he keep her or divorce her?

Guidance to Islam is a great blessing, and whoever is instrumental in it receives a great reward; "For by Allah, that a single man should be guided through you is better for you than red camels." Therefore, one must be keen on guiding one's wife to Islam due to the goodness therein. As for her remaining steadfast in Islam, that matter is with Allah, "Indeed, the hearts are between two fingers of the fingers of Allah; He turns them however He wills." It is preferable to be optimistic about this; "And I am pleased with good omens... a good word." If the wife apostatizes after embracing Islam, then separation is obligatory until she returns to Islam.