They will reap the whirlwind. The most meager knowledge of history makes it plain that human nature is so depraved that without continuing and constant education in morality, it sinks to the level of the brute. Standards have been equated with legalism, and ridiculed. Literally millions of professed Christians will go to hell with puny standards. The fact that those claiming to believe in the infinite righteousness of Christ have simultaneously cherished a shocking low-level of practical righteousness in their own lives, proves the deceitfulness of sin.
Human nature is proverbially one-eyed.
Some people can read the first half of every letter Paul wrote about salvation by grace alone, yet ignore or misunderstand the second half of those same letters that call for the dedication of body, soul, and mind to the Redeemer. Often confused behavior is rooted in confused thinking. This is true of professed Christians who live with low moral standards. Their grace is a cheap grace, not true grace. They may mouth New Testament texts against the law, but reveal a colossal ignorance of what the texts are really affirming.