What sincerity means..

Sincerity means more than mere honesty. It means that you mean what you say, feel what you profess, are earnest in your will. As the sadhak aspires to be an instrument of the Divine and one with the Divine, sincerity in him means ‘that he is really in earnest in his aspiration and refuses all other will or impulse except the Divine’s.

Ref: Guidance from Sri Aurobindo – Letters to a young Disciple