Has Paul given believers an understanding of the mysteries, and why do so few have it?
Yes. Paul has given every true believer an understanding of the mysteries. The problem is that most believers do not have that understanding, so it must be taught and clarified. The context does not state a further reason why so few possess it.
Les teaches that Paul has given to every true believer an understanding of the mysteries, but the problem is that most of them do not. He says he has almost made it a priority to help believers understand what these mysteries are that Paul is always referring to. He pictures them as a great big wagon wheel with spokes, the various mysteries making up what he calls the Body of Truth, which Paul calls the dispensation of the grace of God. Within that dispensation are Pauline doctrines not found anywhere else in the Bible. He then plans to run through Scripture, starting in Romans 11:25 and moving through Paul’s letters. The context does not give a further explanation for why so few have this understanding.