Discerning things which differ
Christian teaching has to be evaluated. Much of the content New Testament epistles was full of warning against specific kinds of teachings that would distract believers from Christ, rob them of their stand in grace, and bring them into bondage to men. 2,000 years later, the same warnings are still relevant. The trends that were emerging in a “seed” form among the little churches that existed in Paul’s day have now blossomed into fully formalized systems of teaching (systems of error as Paul called them in Ephesians) that need to be contrasted with the grace teachings of the Apostles if we are really to stand in our freedom in Christ.
Yes you “really” believed when you believed.
One of the tactics that Satan uses to shake people’s confidence and plunge them into self doubt and condemnation is to question whether or not they really believed the gospel when they believed it.