"Sincere faith" refers to a genuine and honest belief in something or someone. It means having a deep conviction and trust with no pretense or ulterior motives. Full definition
That's true whether you see sincere faith in its heart or not. (moviemezzanine.com)
If sincere faith is not present, go back to # 7. (redeeminggod.com)
All this being granted, Father Vagaggini said, it would appear that one can accept a further hypothesis: There is nothing to deny the fact that in the non-Christian religions even after Jesus Christ there have been elements, in greater or lesser number — whether of a ritualistic, institutional or doctrinal nature — with a positive value for salvation, in the sense that God makes use of them to effect the salvation of those persons of sincere faith who belong to these religions. (religion-online.org)