Praise be to Allaah.
The best thing for you to do is to become Muslim, because Allaah created mankind to worship Him as He wants. He wants us to worship Him in the Islamic way. The Christian religion was right in its own time (when it first came), but it has been distorted, then Allaah abrogated it with Islam. The Muslims recognize the religion of the Messiah, may peace be upon him, but it was followed before Islam came, by a small group of people. Then Islam came and abrogated it, so it is not right to follow it now.
After man dies, he will have another life. Either he will be blessed (in Paradise), if he worshipped Allaah as Allâh wants, or he will be doomed (in Hell) if he went against the commands of Allaah Who created him and gave him his provision. So how can you worship Allaah by following an abrogated religion? We advise you to read good Islamic books, and you will be guided to the truth, by the permission of Allaah. Do not be deceived by the large numbers of Christians around you, or by the fact that it is the religion of your forefathers, or by the influence of the media. You have a mind of your own, so use it and do not live according to what others think.
If you insist on remaining Christian, then in Islam it is permissible for a Muslim to marry a Christian woman, if she is chaste and does not engage in forbidden relationships. It is not permissible for him to live with her outside of marriage. If he does that, then he is a sinner who has disobeyed Allaah by committing a serious major sin, and he deserves punishment in this world and the next.
We ask Allaah to guide you and him, to make you strong and to grant you a good end. Allaah is the Guide to the Straight Path.
Sheikh Muhammed Salih Al-Munajjid