For Paul, one way that ethnic particularity is expressed is by not doing (gentiles) or doing (Jews) the Mosaic Law. Though that Law-keeping (or not) does not save, it marks them out as those whom God has saved as Jews (or gentiles). Said differently: such Torah-observance by Jews did not earn their salvation, but it was the expected practice of Jews whose Lawkeeping expresses their Jewish nature, which God did save. That is, God through Christ is saving gentiles as gentiles, and he is saving Jews as Jews. 

Consequently, gentiles ought not Judaize by circumcision, and the logic that compels Paul to prohibit gentile Judaizing by circumcising specifically and whole-Torah-observance generally, which is partially based on God’s promise to save the gentiles qua gentiles, likewise compels him to expect Jewish Law-observance as a second-order good that expresses their Jewish nature in view of his conviction that God will justify “the circumcision” and those “of the Law”, i.e., Jews, as Jews. - Paul T. Sloan, Jewish Law-Observance in Paul