Being sincere is not the way to have your sins forgiven. If it were, then Jesus didn't need to die on the cross. But He did die, so being sincere isn't good enough. Besides, if someone appeals to their own sincerity as a worthy effort before God, then they are appealing to their own character as a means to satisfy an infinite and holy God. But this is impossible. First, we can never satisfy an infinite and holy God based on our finite sin-stained efforts and second; making such an appeal is really an appeal to pride since it is an appeal to something good in ourselves. The problem is that we are all touched by sin. The Bible says, "no one does good," (Rom. 3:12).
Unfortunately, people erringly think that having a good and sincere heart is sufficient for them to escape God's judgment. But this is a misplaced appeal since it is based on feelings and not Biblical revelation. Furthermore, someone can be sincere but be very wrong in what they believe. Since we are justified by faith (Rom. 5:1), we must believe in the true God since faith in something false is useless. It doesn't matter how sincere you are because sincere belief or faith in something false can't help you. People are sincerely wrong all the time. Does anyone expect God to overlook the necessity of the sacrifice of His Son based on someone saying on judgment day, "But God, I was sincere." No, it won't happen.
So if someone is sincere yet believes in a false God like the Mormons or the Jehovah's Witnesses do, will they go to heaven? No. They won't. They will go to hell. This is why it is imperative that we preach the truth of God's Word to everyone.