How did Paul receive the unique doctrines found in his epistles?
Paul received the unique doctrines in his epistles by direct revelation from God. At his Damascus conversion, he was commissioned to the Gentiles and taken into the desert, where God gave him revelations, as Paul later refers to in Galatians chapter 1.
Les explains that Paul was commissioned right away at his conversion at Damascus, when God told Ananias He would send this man far hence to the Gentiles. God then took Paul out into the desert and gave him His revelations. Les points out that Paul refers to this in Galatians chapter 1, teaching that by revelation God made known unto Paul all these things that would come from his pen and that you won't find anywhere else in the whole Bible. Les also notes that Paul said all Scripture is inspired of God and profitable, and that Paul never lifted himself up as the only one to pay attention to, yet he brought out many doctrines not found elsewhere in Scripture. Therefore, Les says it behooves us to spend the best part of our time in Paul's epistles and from that funnel back and forth throughout all the rest of the Scriptures.