Yes, he is teaching that. The context is justification. They were falling from the justification they had in Christ and returning to a reliance on justification by works of the law. The epitome of which was physical circumcision, the sign of the covenant (Sabbath keeping being the other 'sign' observance).
I'm sorry I don't see where he was talking about justification, what I see him saying is they would lose their liberty.
Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage. Gal 5:1
Look what Paul said,
Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace. Gal 5:4
I ask, who was ever justified by the Law, no one, that's one of the reason's I believe he was speaking hypothetically.
If that were true, why did God command Abraham and countless saints after that to be circumcised? Timothy was circumcised by no less than Paul himself.
Abraham is not the Church, and good point, ...yes Timothy was circumcised by Paul, did he fall from grace afterwards?
Again, why would Paul circumcise his convert and protege if he knew he would fall from grace, ...unless he was speaking hypothetically to the Galatians.
Circumcision, for the purpose of being justified (see context), would be to abandon trust in faith in Christ alone for justification. That is what is wrong with circumcision. There is nothing wrong with circumcision in and of itself. Paul circumcised Timothy. That alone shows us it is not a sin, in and of itself, to get circumcised. Circumcision is wrong when you are doing it to earn a declaration of righteousness, instead of trusting Christ alone through faith for that declaration of righteousness.
I understand your point and I agree with you in faith in Christ only, but I reiterate, I believe Paul started teaching them in verse 1 with the idea of them losing their liberty.
And thus the age old question: Can the Galatians really continue to turn away from their faith in Christ until God turns them over to that decision, or is the warning sufficient enough to bring them back? Hebrews 10:29 NASB answers that question for me.
And that is exactly the point of this thread, an age old question that is never asked in the Word, ...so then men have to give their opinions.