Why are Paul's letters intrinsic to our doctrine?
Paul's letters are intrinsic to our doctrine because he alone was given the revelations concerning the Church made up of Jew and Gentile. He was the Master Builder who laid the foundation, Jesus Christ, according to the grace of God given to him.
Les explains from I Corinthians 3:10-11 that Paul laid the foundation as the Master Builder, and no other foundation can be laid than Jesus Christ. Everything we believe and hope for rests on that foundation of Christ and His finished work of the Cross. Paul's letters are intrinsic to our doctrine because he alone was given these revelations regarding the Church made up of Jew and Gentile. Peter later makes reference to it in his epistles, but in his early preaching he knows nothing of the Body of Christ, this composition of Jew and Gentile. This apostle, however, had all of this revealed to him.