Loving one's enemy is not a New Testament thing. In the Hebrew Scriptures, the Law clearly shows that we are to show love to our enemies in
Exodus 23:4. I try to show love to all, although this can be difficult, if you have love for Yahweh, it's easy to love each other. For Yahweh so loved the world, that he gave his only begotten Son, that whosoever believes on him should not perish, but have eternal life we read in John 3:16. If we want to be like Yahweh, we have to love the unlovely, just as Yahweh loved us despite our sinful conditions and sent us His Son to redeem us.