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Christians are called to love everybody – even those that people would not normally love. This is modelled best by Jesus Christ himself, who loved outcasts, sinners and failures like myself.
This is fraternal love that all of us must have for our common man. When it comes to romantic love, there's slightly more to it.
Christians are asked/commanded earliest in Deuteronomy to not marry unbelievers because they could potentially be read astray. Elsewhere, the bible is riddled with literature about being "unequally yoked" or married to an unbeliever.
That said, we all have free will and can marry or date whomever we want. The question that usually will be on a Christian's mind though, is "Will this please God and be an act of worship to Him?". We also know that being unequally yoked wouldn't please God. Because of this, we choose to seek romantic relationships with Christians only, despite having free will. Why? Because we love God. Why do we love God? Because he loved us.
What happens if a Christian is married to an unbeliever? Are they to leave them?
Pretty much the only time divorce is allowed biblically is when adultery has occured. No adultery, no grounds for divorce, even if it's an interfaith marriage. The Christian party is asked to love and serve and lead a holy, God-pleasing, God-glorifying life as an example to their spouse.
What about dating? Can my Christian girlfriend dump me because I'm not a Christian?
Yes, she can. She can become aware of how the relationship may be getting serious, leading toward marriage, thinking about if the marriage will please God, realizing it won't, and then ending it.
The thing about Christianity is, God comes first, no matter what, no matter how hard it can be. Why? Because to him, we come first.
I hope that was helpful!