Thanks for your question. What a tough situtation! I’ve got to say at the start that I don’t know you very well, nor do I know your friend, so I’m going on guesses and generalities. So if at any point it feels like I don’t really understand your situation, you’re probably right, but I do think I can say something helpful nonetheless.
First, let’s think about why Christians and non-Christians shouldn’t enter into exclusive relationships (dating). Dating can serve a variety of purposes. But primarily, for Christians at least, dating is about heading towards marriage. I’ve once heard someone say that we either enter into an exlcusive relationship with someone with the intention of marrying them or breaking up with them. One is the proper goal, the other is a cruel goal. (Now we may break-up having begun with the intention of heading towards marriage, that’s fine and a part of the temporary nature of dating). In many ways, marriage is about the private and exclusive relationship going public and permanent, rather than private and temporary. And that is the goal of Christian dating.
So, to enter into a dating relationship is to begin on the track towards marriage with someone. So the next question is ‘Can Christians marry non-Christians?’ This is a very modern question that most Christians in the time of the BIble had little control over. You were betrothed to someone from a young age and you married them. The concept of choice in marriage only applied to a very small number of people. And there is only one verse in the BIble that gives recommendations to this small group of people - widows. 1 Corinthians 7:39 A woman is bound to her husband as long as he lives. But if her husband dies, she is free to marry anyone she wishes, but he must belong to the Lord. - The apostle Paul tells us that a widow is free to marry anyone they want, ‘but he must belong to the Lord.’ The widow is in the same situation as most modern Christians - free to choose. Indeed a widow’s plight may be more drastic than most, in an age with no income support or welfare, widows would often have no choice but to marry if they wanted to eat! They needed to find someone to marry as a matter of life or death and Paul says - he must be a Christian! Why? I think its a wisdom issue, but also, Paul asserts this as someone speaking for God! God commands single Christians to only marry Christians. The wisdom in the command is probably something you experienced when you were going out with your friend. When two people enter into a relationship, they bring their worldviews together and must agree to walk in the same direction with their lives - otherwise it just doesn’t work. A Christian lives their lives oriented towards the Lord Jesus, non-Christians hate the Lord Jesus (the Bible is pretty clear on that one, as nice as non-Christians often are). The two lives are just not compatible! Living with a non-Christian will limit your ability to serve, to give, to go to Church, to offer Christian hospitality, and you will find yourself constantly having to decide between two people you love - generally speaking one will eventually win out over the other, either you will end up hating Jesus and loving your non-Christian husband, or hating your husband and loving Jesus. Neither option is great! Being married to a non-Christian is a long, lonely, painful existence. God gives us commands for our good, he does love us!
Now I suspect you know all this, you dated your friend before and probably experienced (even if only in a small way) some of the pain that life means. You are struggling with your decision to not go out with your friend, so you must have thought about these issues before. I just wanted to point out with the above that your decision is actually a good one, it’s not just obeying some silly rule (which your friend no doubt feels). So, what strategies can you follow to avoid the horrible life of being married to a non-Christian. (I’m not in your situation, so I can’t tell you which one is best for you - you may be too close to the situation as well, you may need to ask someone you trust and who is spiritually mature, maybe a pastor or a Bible study leader, for their advice)
1. End it. You may find that your feelings are just too strong for you to trust yourself with this guy. For his sake and yours, it may be best to finish the relationship altogether. A quick, clean break heals much faster than a prolonged, messy mistake. If Jesus is truly Lord in our lives, we often have to say ‘no’ to things even when they feel very good! It would be helpful to have someone to support you through this as there will be a painful period of mourning for the loss of the relationship. They can also keep you accountable, so that you don’t “relapse” into the relationship again.
2. Change the circumstances in which you relate to your friend. You will find it much easier to not get into serious relationship questions if your relating is exclusively in group settings. People going out spend lots of time alone together are almost definitionally dating (though it’s probably not a great idea to be so alone together when you’re dating anyway - but that’s another topic for another day). If your pattern of relating looks like what dating people do (going out alone together, hanging out at home together alone etc) then it’s no surprise your friend still wants to consider you as dating. It may help to be very clear. You are no longer dating so it is not appropriate for you to be alone together any longer. Keep your times together as group times, with others. You will need to demonstrate with actions the words you are saying to him.
3. He becomes a Christian. If your friend becomes a Christian, you are free to marry him (1 Cor 7:39), so you are free to date him. However, you will of course want to be sure that he has changed for Jesus and not for you (because in that last case the change back is super easy). Just a word on him becoming a Christian. It is very, very, very, very, very, very, very, very unlikely to happen a) by him hearing about Jesus exclusively from you (there is too much confusion in your relationship for him to clearly understand Jesus); b) unless you demonstrate how important Jesus is to you by showing him that you are willing to give up on your relationship with him for the sake of following Jesus - Jesus is that important - your friend needs to deal with him too. So get other Christians involved in his life, I reckon you need a Christian bloke to meet up with him and explain from the Bible who Jesus is if he is to do any serious business with God. Guys will do just about anything for a girl they’re interested in, but you want it to be real!
I think you need to not give him false hope that if he meets up with someone, if he goes to church, if he becomes a Christian, then you will go out with him - it may just tempt him to lie to you to go out with you. Instead be clear that because he isn’t a Christian, you can’t go out with him and in your loving of him, encourage him to find out about Jesus. Keep the two seperate so neither of you kid yourselves.
In the end, as nice as he is, as attracted to him as you may be, if he cannot respect your alliegence to Jesus, he doesn’t respect you. You and Jesus are a package deal! Stick to it, find some support from trusted Christians and I reckon, think seriously about the first two strategies and pray for the third. Our God is good and kind and merciful, throw all your cares upon him.
I hope this has been helpful. I’d also like to suggest you get a copy of Nigel Pollock’s book ‘The Relationship Revolution’, you might find it helpful as well.
PS As trite as it might sound, can I recommend looking around at the Christian men around you. There are some cracking Christian guys who would be great “catches”. They are often shyer and not as cool as non-Christian guys can be, but if they love the Lord Jesus, they will try to love you the way Christ does - there’s nothing better than that! (Please see this as advice for later, once the pain has been dealt with)