Is marriage to an atheist woman considered void and forbidden, and must she be divorced?

What you have committed by having an illicit relationship with that girl is a grave evil, for zina (fornication/adultery) is among the most heinous sins, and you must hasten to sincere repentance. Your marriage to this girl is not permissible due to the zina that occurred between you, and because she is not chaste and is pregnant from an illicit act, and because she is an atheist. Therefore, it is obligatory to separate from her immediately. And be keen to marry a woman of religion and good character.