Imperative singular, appealing to each individual reader.
I Paul
Comp. 2Co 10:1; Eph 3:1; Col 1:23. Asserting his personal authority.
If ye be circumcised (ἐὰν περιτέμνησθε)
Better, receive circumcision. The verb does not mean that they have already been circumcised. It states the case as supposable, implying that they were in danger of allowing themselves to be circumcised.
Christ will profit you nothing
Circumcision is the sign of subjection to the Jewish “yoke” - the economy of the law. The question with the Galatians was circumcision as a condition of salvation. See Gal 2:3, Gal 2:5; Act 15:1. It was a choice between salvation by law and salvation by Christ. The choice of the law involved the relinquishment of Christ. Comp. Gal 2:21. Chrysostom says: “He who is circumcised is circumcised as fearing the law: but he who fears the law distrusts the power of grace: and he who distrusts gains nothing from that which he distrusts.”