And, as an evil tree cannot bring forth good fruit, so a good tree cannot bring forth evil fruit. Therefore, as the heart of a believer is, so likewise the believer's life shall be. It includes every motion of our heart, of our tongue, of our hands, even our body language. If therefore thine eye be single, thy whole body shall be full of light. We should have a single intention, namely to do God's will.
Sincerity is about carrying out our intention.
Sincerity means that we actually do what we aim at by simplicity. We walk continually on in the highway of holiness, in the paths of justice, mercy, and truth. If you think about it all people are sincere about some things. Sports fans are very sincere about their particular team. Some people are very sincere about their political party.