It appears that to many, including some evangelicals, God will open the doors of heaven to all who were sincere in life, even if they were sincerely and profoundly wrong.
However appealing such an idea is at a disneyesque level, it is wishful thinking. The Bible never views sincerity as a basis for eternal life. Those who crucified Christ were 'sincere' in their intent. Paul, the Apostle, with zealous sincerity persecuted the early church.
Jesus reminded his disciples that the days would come when, 'anyone who kills you will think he is offering a service to God.' (Jn 16:2).
Indeed, Paul speaks of the sincere but misguided zeal of C1 Israel for God; 'they are zealous for God, but their zeal is not based on knowledge.' (Roms 10:2)
However, Israel's sincere zeal and the sincerity of those who would persecute the church was anything but a saving sincerity or saving zeal. Paradoxically, their 'sincerity' stemmed from a failure to 'submit to the righteousness that comes from God' (Roms 10:3).
Sincerity does not save; in fact all 'sincere' belief that does not flow from trust in Christ, is sincere rebellion against God. Sincerity in itself is of limited worth. Fascists are sincere. Islamic terrorists are sincere. Pro-abortionists are sincere. Atrocious acts are perpetrated by sincere people.
Jesus came to save, not the sincere, but sinners. Sincerity cannot save, but it may well damn.