Paul said in Romans 7, "So I find this law at work: Although I want to do good, evil is right there with me." (NIV)
We were born with a sin nature. We are sinners. Because God loved us, he sent his Son to die on the cross for us to free us from the final consequence of our sin (Romans 6:23). But for a reason, he let something the Bible calls the Flesh to remain in us.
"For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not." Romans 7:18.
We try and try to be good....but eventually fail. We have the flesh in us that keeps us from pleasing God. The flesh can't produce anything that pleases God. "In my flesh dwelleth no good thing.
But God in His mercy, provided a way out. When Jesus died in the cross, we were crucified with him as well!
"I have been crucified with Christ [that is, in Him I have shared His crucifixion]; it is no longer I who live, but Christ lives in me. The I now live in the body I live by faith [by adhering to, relying on, and completely trusting] in the Son of God, who loved me and gave Himself up for me." Galatians 2:20. (AMP)
We must believe that the flesh was crucified, day by day. We can't fight the flesh, God almighty took care of it in the cross. "Christ lives in me". The growth is not going to happen overnight. It is a long process, but although we might not be noticing, it is happening. Christ will reproduce his life in us, if we let him. We are free from sin.