[T]o those within legalistic systems, legalism is a refuge from the insecurities of life and the uncertainties of our world.This sort of preaching "get right with God" misunderstands justification, sanctification, substitutionary atonement, and the finished work of Christ. It often creates a false system of worship—a set of standards that depraved humans are capable of reaching. Even more scandalously, this system necessitates a new god—a god small enough to be satisfied by it.
This is one of the reasons why it is so difficult to talk someone out of a legalistic church. There is so much “certainty” and comfort in knowing exactly what one must do to remain in “right with God.” Legalism requires so little faith, because every aspect of life is defined and mandated. In contrast, the concept of grace and Christian liberty is a scary wilderness of uncertainty. Better to stay in the fortress (or prison). . . .
For anyone who has ever lived in a legalistic system, this sounds all too familiar. The Fundamentalist variety of today would never deny that salvation is all by grace, but the not so subtle message is that to be “right with God” requires the keeping of the rules.
Seems to me that a heresy doesn't get much more fundamental than that.