To be entirely sincere means to desire the divine Truth only, to surrender yourself more and more to the Divine Mother, to reject all personal demand and desire other than this one aspiration, to offer every action in life to the Divine and do it as the work given without bringing in the ego. This is the basis of the divine life.
One cannot become altogether this at once, but if one aspires at all times and calls in always the aid of the Divine Shakti with a true heart and straightforward will, one grows more and more into this consciousness.
Ref : The Mother with Letters on The Mother