Why should Christians know the Bible forwards and backwards?
Christians should know the Bible forwards and backwards so counterfeit teaching becomes obvious immediately. When they know what Scripture legitimately says, error cannot hide; it pops right up, and they see it right away. This thorough knowledge protects them from deception.
Les explains that the same principle used for recognizing a counterfeit applies to the Word of God. If people simply know what the Bible says and know it forwards and backwards, then when counterfeit stuff comes along they will see it right away. That is really his main purpose. In the surrounding context, he also returns to Galatians, reminding listeners that Paul proves we are not under the Law but under Grace, an example of the legitimate teaching believers need to recognize.