The way you can tell what is a forgery, is by studying the real thing. Yet, the main impact of acknowledging your need of a Savior from a sinful condition, is the indwelling of the Holy Spirit after your rebirth conversion. The Spirit if God lives inside the Christian. He leads you into all truth. If you have someone that has claimed to be a Christian that converts to another religion, they were never born again. They may have had a mental assent to relating to the teachings of Christianity, but never truely gave their life to Christ.