Why would the circumcision of the Galatians result in the Jewish Christians (Judaizers) boasting in their flesh?
The circumcision of the Galatians would have been seen as an accomplishment by the Judaizers. If they set out from Judea with the plan of making sure that Gentile believers were circumcised, they would have been able to return home with news of their success. In this sense, they could have boasted in the flesh of the Galatians, boasted about how many people they had been able to convince to get circumcised. To their minds, it might even have served as an accomplishment to bring before God, as if it is what God has wanted from them. 1
13 Car les circoncis eux-mêmes n'observent point la loi; mais ils veulent que vous soyez circoncis, pour se glorifier dans votre chair.