The heirs of God's covenant were not treated as sons before Christ came to redeem us.
Why go back to that treatment, under the law, observing its days?
You followed me so zealously at first; what has changed? The Judaizers exclude you to try to get you to follow them.
Abraham had two sons, and they represent two covenants: slave and free. We are children of promise, like Isaac, born of the Spirit and not under the law. Never mind the children of the slave woman, who persecute you like Hagar persecuted Sarah. The Jerusalem above is our mother and we are free; we are not of the Jerusalem below, Judaism, of Mount Sinai, of bondage.
If you accept circumcision after believing in Christ, you're relying on the law to save instead of Christ. This forsakes Him and puts you back in bondage. Stay free, and use it to love others, not gratify sinful desires. Walk in the Spirit and yield His fruit.
Restore gently those caught in sin, and don't sin yourself while doing it. Help each other while bearing your own load. Give to teachers and those in need, especially in the church. The Judaizers persuade you to boast in how many are circumcised; I'll only boast in Jesus' cross. Circumcision isn't what counts, but being a new creation in Christ - all such are the Israel of God.
How this is about Jesus
Jesus makes us free. By Him, we are made sons, not servants, or guardians under age.
We should not seek out Old Testament shadows to rely on when He has come.
They may continue to teach us about Christ, but are not to act as we are under them.
Our only reliance for our good standing with God is Jesus Christ. Freedom in Him yields righteousness and walking in the Spirit.
Faith in Christ is the defining marker of God's people, not circumcision.
Don't be "cool-shamed" into following worldly or legalistic things. Both use this tactic, excluding you until you follow their ways. Never mind them, if they are wrong!
Reliance on yourself to be right with God usually leads to corruption.
Self-sufficiency and generosity are marks of God's people.