Galatians 3:3. Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?
We have been given the zoe life of God. His very own divine life, by the Spirit, lives in us in the person of Jesus Christ. This indwelling life of Christ, our union with Christ, is what perfects us. Never the flesh.
What is the flesh? “Flesh” is the Greek word sarx. Sarx can mean the actual physical body, but for Paul it is usually used to mean the human condition. It is the sum of our human achievements and efforts. Often it takes on a negative tone, because Paul will talk about our flesh being stirred up by the law and thus leading to sin. In other words, the flesh is what causes us to want to succeed, but apart from grace- by our own self efforts.
Here in Galatians 3:3 Paul is calling out the stupidity of beginning by the Spirit of Christ, by grace, by an indwelling life and love, and then turning from that grace to your own human achievement. I mean who would do such a thing? Only a complete fool (like you and me and Lego Batman).
This is of course, because by our own human effort, our flesh, we can never achieve perfection. Also the flesh is pursuing something that has already been freely granted. Christ became flesh. He took on the human condition and accomplished all human achievement. By our union with Christ, we have perfection, we have holiness, we have all of Christ’s righteousness, we have his sarx. When you try to obtain these by your own good deeds, commitments, dedications, rituals, penance, and law keeping, you are rejecting the Tree of Life for the Tree of Knowledge. You are rejecting abiding in Christ. You are rejecting zoe. You are rejecting Christ himself.
“To live is Christ” means there is no need to run back to the flesh. It is constant resting in the finished work of our Savior King. Only a fool would rely on their own self efforts over the perfect effort of the Christ.