All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The marriage of a Muslim woman to a Christian man is void and invalid. Moreover, according to the consensus of scholars, it is not effective at all. Their consensus is based on the verse in which Allaah The Almighty Says (what means): {And do not marry polytheistic men [to your women] until they believe.} [Quran, 2: 221]
Consequently, this marriage is considered Zina (adultery/ fornication) and immediate separation is due. A Muslim who considers this marriage lawful, despite knowing the relevant Sharee’ah ruling, is an apostate. Being married to a Christian does not make her apostate unless she considers it lawful. This is applicable if she abides by the rulings of Islam and performs its apparent rituals, most importantly prayer. Also, she must not be committing anything that invalidates her Islam.
If she hates Islam, loves the Christians and has allegiance to them, she is an apostate. The same ruling applies if she commits any other acts that invalidate her Islam.
Allaah Knows best.