Can one be sincere and wrong?
Some think that God is like the top of a mountain -- there are many paths to the summit. What's important is not the particular path but that you're climbing. However, one can sincerely climb the wrong path and fall off a cliff.
More than once a wrecking company has efficiently destroyed a house only to find they had gotten the wrong address -- sincere, but wrong. We can be 100% sincere, yet still be 100% wrong. Sincerity is important, but it doesn't determine truth.