"Now how can we do this? Where does power to love like this come from? Just think how astonishing this is when it appears in the real world! Could anything show the truth and power and reality of Christ more than this?
Let me just give you part of the answer from Matthew 5:11-12,
Blessed are you when men cast insults at you, and persecute you, and say all kinds of evil against you falsely, on account of Me. 12 Rejoice. and be glad, for your reward in heaven is great, for so they persecuted the prophets who were before you.
Jesus says that not only can you endure the mistreatment of the enemy, but you can also rejoice in it. Why? Because your reward in heaven is great.
Which means that the command to love your enemy is a command to set your mind on things that are above, not on things that are on the earth. The command to love your enemy is a command to find your hope and your satisfaction in God and his great reward--not in the way people treat you. The steadfast love of the Lord is better than life (Psalm 63:3).
Loving your enemy doesn't earn you the reward of heaven. Treasuring the reward of heaven empowers you to love your enemy."
This excerpt is taken from John Piper's sermon "But I Say to You, Love Your Enemies, Part 2." Click over here to see the rest of the article.