A Christian man is married to a Christian woman but he wants to divorce her and marry a Muslim girl. He has a daughter who was born in the Christian marriage. If he became a Muslim he will lose his daughter. What is the solution? Does his daughter become Muslim when he changes his religion? She is 3 years old.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

One should advise this person to hasten to embrace Islam before facing his death. If he dies in the state of being a non-Muslim, he will surely enter Hellfire forever. Allah Says (what means): {Indeed, he who associates others with Allah - Allah has forbidden him Paradise, and his refuge is the Fire. And there are not for the wrongdoers any helpers.} [Quran 5:72]
His daughter should not be an excuse to delay embracing Islam since his Christian wife will remain as his wife if she is chaste. Allah permitted Muslims to marry the chaste women from the People of the Book. The daughter follows the parent who is upon the best religion; i.e., Islam.

If the rules of his country do not allow him to keep his Christian wife or the wife will not accept him after he embraces Islam, he should accept Islam as soon as possible and spare no efforts to save his daughter. If he cannot do so, then he should take care of his daughter till she becomes mature. And then he should convince her that she is a Muslim due to the religion of her father and should follow the teachings of Islam.

Moreover, he should consult the Muslim organizations and Islamic centers in his area to seek their help.

For more benefit on the religion of children when one of the parents is Muslim and another is not, please refer to Fataawa 81957, 86359, and 91184.

Allah knows best.