Romans 6:17-18 (KJV) (17) But God be thanked, that ye were the servants of sin, but ye have obeyed from the heart that form of doctrine which was delivered you. (18) Being then made free from sin, ye became the servants of righteousness.
We were once slaves to the sin nature. That was before we were saved. But, when we heard about Jesus and Him being crucified, we became children of God. This doctrine of being saved by the blood was first given to the Apostle Paul, who then gave it to us through the Epistles he wrote. Jesus set us free from the sin nature. As long as we continue to have faith in what was done at Calvary, the sin nature no longer has power over us. Because of that faith, we are now slaves to righteousness, and are constantly pulled toward it.
(Read 1 Peter 4:2)