Romans 11:5,6

"In the same way then, there has also come to be at the present time a remnant according to God's gracious choice. But if it is by grace, it is no longer on the basis of works, otherwise grace is no longer grace."

Insidious. adj. "operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect"-- Random House Dictionary, 2010.

Insidious comes to mind whenever considering systems of theology that speak of grace in terms other than the biblical usage, characterizing it as a means of meriting favor (salvation). Various cults, such as Christian Science, Church of Jesus Christ of Latter-Day Saints, Jehovah's Witnesses, etc. teach such heterodoxy, a "gospel" which is not good news at all.
These groups' popularity speaks to the natural tendency in lost folk to believe that surely some good works will help earn a spot in heaven. Still, these cults' influence is not as insidious, nor as pervasive, as the consummate organized "works-'grace' " theological system today: Roman Catholicism.

With its sacramental system inherently works-based, the entire theological structure is skewed, because the foundation is faulty. What is this foundational fault which 'proceeds in an inconspicuous or seemingly harmless way but actually (has) grave effect'?
AUTHORITY. By asserting that its authority (Church Tradition) is on an equal plane (superior, actually) with Scripture, the Roman Catholic theology of "salvation-by-sacramental-obedience" subverts the true biblical meaning of grace.
For, to add any effort whatsoever to the freeness of the genuine Gospel's grace of Scripture is to invite such questions as, "How much effort is enough? How many good works does it take to know you have salvation? Where is the joy of salvation's assurance, especially at death, if one is depending on works for acceptance?"

The implications are enormous, important, and of eternal consequence. Why? People act on what they believe. And those who teach them are promised by God a stricter judgment (James 3:1). Those who teach the systems of works-salvation in all their insidious forms will have much to answer for, when before the King (Romans 14:12).