Sincerity of belief does not justify the belief. This is why we must seek truth, which is something objective and absolute, not subjective.
“A drunkard reaches a point at which (for the moment) he actually believed that another glass will do him no harm. The beliefs are sincere in the sense that they do occur as psychological events in a man’s mind. […] But errors which are sincere in that sense are not innocent.” The Great Divorce, C.S. Lewis, 38
Sometimes, we try to justify our acts or beliefs with our sincerity. But if they aren’t in line with God Himself and His character, our sincerity is worth nothing.