What is legalism in this teaching?
In this teaching, legalism is the flesh's desire to return to the Law, an oldness of the letter rather than newness of spirit. It is what the flesh wants in the Galatian churches, part of the old Adamic nature that wants to be under some kind of Law.
Les explains that believers are delivered from the Law, which was an administration of death, so they now serve in newness of spirit and not in the oldness of the letter. Paul is talking about the Law. The Galatians had begun in the Spirit by Paul's Gospel through faith plus nothing, but the flesh wants to move into a deeper spiritual life by the flesh. In the Galatian churches, that fleshly impulse is legalism—Law. It is part of the old Adamic nature to want to be under some kind of Law. The way to enter the spiritual realm is to believe the Gospel.