Is a sexual relationship before marriage considered unlawful, even if it culminates in marriage?

We are pleased with your eagerness to inquire about what is unlawful. However, what is more important is your question about remaining on your old religion despite your marriage to a Muslim. Have you not considered embracing Islam and learning about it? Your conversion to Islam will forgive your past sins; indeed, Allah will transform your bad deeds into good deeds. There is no difference in the prohibition of fornication whether it is followed by marriage or not; fornication is forbidden in itself, and sincere repentance is what rectifies what has passed. If the two who committed fornication repent and wish to marry, there is no harm. However, before repentance, it is not permissible for a Muslim to marry a woman of the People of the Book unless she is chaste and virtuous. If the marriage took place before repentance and without knowledge of the ruling, the marriage contract must be renewed.