How do you answer someone who says they follow Jesus but not what some man like Paul says?
The answer is that Paul did not receive his gospel from any man, but by direct revelation from Jesus Christ. Since Paul's message is not after man, rejecting Paul's teaching is rejecting what Jesus Himself revealed. Paul certifies his apostleship, so following Jesus includes following the Christ-given message through Paul.
Les explains that he loves these verses in Galatians when he defends Paul's apostleship, because he often hears people say, "Well, lest I follow Jesus. I'm not going to go about what some man says." He asks who they think is speaking through this man. In Galatians 1:11-12, Paul certifies that the gospel he preached is not after man. Paul did not pick up someone else's mantle; he neither received it by man nor was taught it by other men, but received it by the revelation of Jesus Christ. Then in Galatians 2:5, at the Jerusalem council around A.D. 51, which parallels Acts 15, they tried to get Paul to back away from his apostleship and embrace Judaism and circumcision along with belief in Christ.
But I certify you, brethren, that the gospel which was preached of me is not after man.