Jamieson, Fausset and Brown on Galatians 5:2

2. Behold--that is, Mark what I say.

I Paul--Though you now think less of my authority, I nevertheless give my name and personal authority as enough by itself to refute all opposition of adversaries.

if ye be circumcised--not as Alford, "If you will go on being circumcised." Rather, "If ye suffer yourselves to be circumcised," namely, under the notion of its being necessary to justification (Ga 5:4; Ac 15:1). Circumcision here is not regarded simply by itself (for, viewed as a mere national rite, it was practiced for conciliation's sake by Paul himself, Ac 16:3), but as the symbol of Judaism and legalism in general. If this be necessary, then the Gospel of grace is at an end. If the latter be the way of justification, then Judaism is in no way so.

Christ ... profit ... nothing--(Ga 2:21). For righteousness of works and justification by faith cannot co-exist. "He who is circumcised [for justification] is so as fearing the law, and he who fears, disbelieves the power of grace, and he who disbelieves can profit nothing by that grace which he disbelieves [Chrysostom].