Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord.
Note 14 at Ro 12:19: Ro 12:19-21 is humanly impossible. It takes the supernatural power of God's faith at work in our hearts to fulfill these scriptures. Letting God be the one who defends us is a matter of faith. If there is no God who will bring people into account for their actions, then turning the other cheek would be the worst thing we could do (see note 22 at Mt 5:44). But if there is a God who promised that vengeance is His and He will repay, then taking matters into our own hands shows a lack of faith in God and His integrity.