Why would a true believer struggle with understanding "Christ on the Cross as forgiveness of sin, that is the only reason God accepts you and lets you into heaven"?
Its because the cult's do not teach this simplicity of Christ. They teach around it, and try to replace it with commandments and works and enduring and mary and any type of self effort they can get you involved in that keeps you from simply trusting that Christ is Salvation, and if you have Him in you, you are going to heaven.