Why were the Galatians tempted to go back under legalism?

They were tempted because Jewish influences pressed circumcision as a necessary addition to the Gospel. This legalism implied Christ's finished work was insufficient, so Paul corrected it, insisting salvation is all of grace by faith plus nothing. Today, similar pressure appears whenever people require other works besides believing the Gospel.

Les explains that the Galatians were going back, or at least being tempted to go back, under legalism. At that time the temptation was prompted by the Jews and centered on circumcision, and Paul dealt with that in particular. The deeper problem is that adding legalistic demands to salvation takes away from the finished work of the cross and implies Christ did not really finish it. Les emphasizes that salvation is all of grace by faith plus nothing else; believers must believe He did everything needed. For today, circumcision is not the criterion, but people still try to add other things besides believing the Gospel.

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.