Vengeance Is God's
Luke 17:2, "It were better for him that a millstone were hanged about his neck, and he cast into the sea, than that he should offend one of these little ones."
God takes the persecution of His children personally. In Acts 9:4 when Jesus appeared to Saul on the road to Damascus and spoke to him about his persecution of the saints, Jesus said, "Saul, Saul, why persecutest thou me?" Saul was not directly persecuting Jesus but he was persecuting His saints. Yet Jesus said, "Why are you persecuting me?" Judgment against those who persecute God's children will not always come in time to prevent their harm but as this warning makes very clear, God will avenge His own (Rom. 12:19).
Letting God be the one who defends us is a matter of faith. If there is no God, who will bring men into account for their actions, then turning the other cheek would be the worst thing we could do. But if there is a God who promises 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.
We are not to take matters into our own hands and defend ourselves.
"Vengeance is mine; I will repay, saith the Lord" (from Dt. 32:35-36; Rom. 12:19; Heb. 10:30). Striving to vindicate self actually shows a lack of faith in God keeping this promise. It also indicates spiritual "nearsightedness" which is only looking at the present moment instead of seeing things in view of eternity.
Even as Christ did not come to condemn the world and is not holding men's sins against them, even so, we have been given the same ministry of reconciliation. For those who do not receive the love we extend to them but rather take advantage of us because of our "turning the other cheek," God will repay.