Love Your Enemies; Trust the Avenger

Paul said that counting on the final wrath of God against his enemies is one of the crucial warrants for why we may not return evil for evil. It is precisely because we may trust the wisdom of God to apply his wrath justly that we must leave all vengeance to him and return good for evil.

Never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” To the contrary, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.” (Rom. 12:19-20 ESV). 

If God does not show wrath, sooner or later we shall take justice into our own hands. But God says, 'Don't. I will see to it.' 
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~ JOHN PIPER
Foreword to the book Pierced for Our Transgressions (by Jeffery, Ovey, and Sach)