Marrying a Christian girl
Q: I am a Muslim guy and I have been in a relationship with a christian girl for a while. I am trying to make this relationship permissible on all aspects and I want to marry her but we are not planning to register this marriage in the court (not legal marriage) just Islamic so that we can be confident that whatever we are doing is not a sin/Haram. We are very serious in this relationship and we want to take it to a legal marriage level but not now since she is still a college student and her parents and not fine with her getting married that early. I believe, I can fulfil all the Islamic marriage requirements from bringing in witnesses to announcing my marriage. It's worth mentioning that we live apart in two different cities, more than 6 hours away from each other but we do see each other every once in a while.
My question is: Is marry this girl according to the Islamic laws permissible?