What was the legalism the Galatians were tempted to return to?
Circumcision was the legalistic requirement pulling the Galatians back under the law, prompted by the Jews at that time. Such legalism demanded works for salvation. Salvation, however, is all of grace by faith plus nothing else, resting in the finished work of the cross.
Les explains that the Galatians were going back, or at least being tempted to go back, under legalism. At that time it was prompted by the Jews, and that was circumcision. Paul dealt with that in particular. For us today, circumcision is not so much the criteria, but it is all these other things people try to tell us we have to do besides believing the Gospel. He emphasizes we are not under the system of law or any legalistic demands for salvation; salvation is all of grace by faith plus nothing else. Adding to it takes away from the finished work of the cross, as if Christ did not really finish it, but He did. Through Paul's writings God lets us know it was finished, and we stand justified, forgiven, sanctified, and glorified by believing it is a finished transaction.