Adam Clarke on Galatians 6:13

Neither they themselves who are circumcised - They receive circumcision and profess Judaism, not from a desire to be conformed to the will of God; but Judaism was popular, and the more converts the false teachers could make; the more occasion of glorying they had, and they wished to get those Christian converts, who had been before proselytes of the gate, to receive circumcision, that they might glory in their flesh. Behold my converts! Thus they gloried, or boasted, not that the people were converted to God, hut that they were circumcised.