Why are Paul's letters essential to Christian doctrine?
Paul's letters are essential because he alone received the revelations concerning the Church made up of Jew and Gentile. As the master builder, he laid the foundation, which is Jesus Christ and His finished work of the Cross. Peter later references it, but early on had no concept of the Body of Christ.
Les explains that the foundation Paul laid is Jesus Christ and His finished work of the Cross, and every man must take heed how he builds on that foundation. Paul's letters are intrinsic to our doctrine because he alone was given these revelations with regard to this Church made up of Jew and Gentile. Peter makes reference to it in his little epistles at the end of his life, but in his early preaching Peter knows nothing of this and has no concept of the Body of Christ; this apostle had all of it revealed to him.