Justification by Faith.
The teaching of justification by faith is what separates biblical Christianity from all other belief systems. In every religion, and in some branches of what is called “Christianity,” man is working his way to God. Only in true, biblical Christianity is man saved as a result of grace through faith. Only when we get back to the Bible do we see that justification is by faith, apart from works.
Once a person is justified, there is nothing else he needs in order to gain entrance into heaven. Since justification comes by faith in Christ, based on His work on our behalf, our own works are disqualified as a means of salvation (Romans 3:28). There exist vast religious systems with complex theologies that teach the false doctrine of justification by works. But they are teaching “a different gospel—which is really no gospel at all” (Galatians 1:6–7).
This might explain why there is little change in a person who converts to Christianity. Everything from there on can be justified by faith.