Is it permissible for a Muslim girl to marry a man who was Christian and converted to Islam, and what is the ruling of the religion on this marriage?

Mere conviction in Islam and acknowledgment of it are not sufficient for entering into it, nor is the person considered a Muslim. This was the case with Abu Talib, who acknowledged the truthfulness of the Muhammadan message but did not embrace Islam. If a man converts to Islam, then its rulings apply to him, and he gains what Muslims gain and owes what they owe, and he is permitted to marry a Muslim woman. However, if he remains a Christian, even while claiming to be convinced of Islam, he is still considered an unbeliever, and it is not permissible for him to marry a Muslim woman, due to the Almighty's saying: "And do not marry polytheistic women until they believe."