Sincerity in that context is no virtue. Christianity loses its character when it makes a virtue of zeal over truth. For example, when this happens, confidence is placed in a pleasant personality making them vulnerable to the failure of personality. They become weak in the moral fiber of truth. Their stability rests upon the ebb and flow of an unstable human being rather than in the eternal Word of God.
Religious zeal is no virtue in itself.
Zeal minus truth is ignorance. Character is always connected to truth. Obviously, a healthy human being trusts people but not in a naive sense. To place our ultimate confidence in a finite human being is idolatry. The grass withers.