Any belief that is not supported by sufficient evidence is immoral. Sincerity is simply a question of whether one really has a belief or not, and makes no difference for this. For why I think that believing without sufficient evidence is immoral, you can read William Kingdon Clifford's essay "The Ethics of Belief" which can be found at:
http://ajburger.homestead.com/files/book.htm
http://ajburger.homestead.com/files/book.htm
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.
