Jamieson, Fausset and Brown on Galatians 6:13

13. Translate, "For not even do they who submit to circumcision, keep the law themselves (Ro 2:17-23), but they wish you (emphatical) to be circumcised," &c. They arbitrarily selected circumcision out of the whole law, as though observing it would stand instead of their non-observance of the rest of the law.

that they may glory in your flesh--namely, in the outward change (opposed to an inward change wrought by the Spirit) which they have effected in bringing you over to their own Jewish-Christian party.