How often did you read a letter signed with, sincerely yours? Or how often did you hear that is a sincere person? Both are common in our language. The use of the word sincere is so common that people hardly ever ask for the meaning of it anymore. It has become a word that lost its power in the opposite meaning. Sincerity is drawn in insincerity.
To restore the meaning and the value we should first understand the power and quality of the meaning and begin to use it for ourselves.