Look out for the dogs, look out for the evildoers, look out for those who mutilate the flesh.
For we are the circumcision, who worship by the Spirit of God and glory in Christ Jesus and put no confidence in the flesh—
Paul uses strong and negative words to describe a people and religion he once zealously promoted and defended. He is still zealous, but is now walking with Christ rather than persecuting those who believe in Him. He has put aside childish things. He no longer needs a guardian to care for him until he matures.
He once saw himself as blameless. That is common among the self righteous. Confidence in the flesh can manifest in the boldest, most ostentatious, overt display, to a subtle whisper in the ear of the the most pious believer. Both have a negative effect on the heart.
Circumcision in the flesh was an intermediate step which would serve as a reference point that would be used to describe and explain a mark that each believer would receive. The Spirit of God is the ultimate mark that is the sign of our inclusion in the house of the Lord. It is through the Spirit we are able to embody the kind of worshippers sought by the Lord.