Why is it a problem to only go by what Jesus said in the four Gospels?
It is a problem because restricting oneself to the four Gospels confines believers to Christ’s earthly ministry before His death, burial, and resurrection. At that time the Twelve did not know He would die or be raised. God later revealed the gospel to Paul directly, not through men, by the risen Christ in glory.
Les explains that people often say they will not go by what a man like Paul says but only by what Jesus said. As a result, they keep everything they teach and preach back in the four Gospels and Christ’s earthly ministry. But that ministry was before His death, burial, and resurrection; it had not yet happened. The Twelve did not even know He was going to die, and after He was crucified they had no idea He would be raised. That was something God was going to reveal especially now to this Apostle. In Galatians 1, Paul certifies that the gospel he preached is not after man; he neither received it of man nor was taught it, but by the revelation of Jesus Christ, who is now in glory.