Is it permissible for a married woman to marry a Christian man, and what is the ruling on one who does so?

A Muslim woman's marriage to a non-Muslim is forbidden by the Qur'an, Sunnah (definition), and scholarly consensus, whether the non-Muslim is an idolater, a Jew, or a Christian. This is based on the Almighty's saying: "Then do not return them to the disbelievers; neither are they lawful for them, nor are they lawful for them." And whoever does so is an adulteress. As for a married woman marrying another man, it is even more forbidden and heinous.