The Gospel Is Not Man-made
What are we meant to learn from Galatians? Some say the primary lesson is Paul's authority as an apostle, and that certainly is a clear and important point Paul makes repeatedly. But that is not Paul's chief goal with the letter. His main goal is that – by trusting the divine authority of Paul's apostleship and message – his readers would be enabled to embrace the grace of God through faith in Jesus Christ.
Paul wants his readers to know/trust that the totally counter-intuitive, other-worldly message he is proclaiming is not a human idea. No, grace is a miracle. So we as Christians are walking miracles every day!
Why would we give up the miracle for the mundane? Why replace the real and divine message with a false and man-made one? Why turn away from free grace through Jesus to the endless slavery of succeeding on your own?
God, for his own glory, sent Jesus into the world to save people, like you, who could not save themselves. Why reject such wonderful news?