The false gospel of legalism says (perhaps implicitly) that behavior modification is the way to salvation. It forsakes the root: the sinful human heart. Behavior modification and a code of morality does nothing to transform the heart and mind.
You can fix the appearance of a house but if it has a rotten foundation, it will not stand despite how pretty it looks.
The real gospel takes an axe to the root: sin, an internal disease that manifests itself outwardly. Addressing the root allows for change at the most fundamental level, at which the cracked foundation can be replaced with one that is solid, one that will hold: with Jesus. Upon Christ, the Solid Rock, something that lasts for all eternity can be built. He is forever unchanging; strong and mighty, and no storm can tear Him down.
Let us not bother with preaching mere morality, but with preaching Jesus. Outward change is a result of inward transformation. Works as a RESULT of a transformative faith.