Therefore, if anyone is in Christ, he is a new creation; the old has gone, the new has come! 2 Corinthians 5:17

God promises that when a new believer begins his new spritual life He will give us a new heart. Nonetheless, we still have the capacity of sin. The penalty for sin has been broken, but sin's potential has not been fully removed. Even after we accept Christ, our old nature battles within us. Sin comes naturally to humans. Our new nature will prevail every time we obey God and resist sin.