Sincerity is perhaps the most difficult of all things and perhaps it is also the most effective.
If you have perfect sincerity, you are sure of victory. It is infinitely difficult. Sincerity consists in making all the elements of the being, all the movements (whether outer or inner), all the parts of the being, all of them, have one single will to belong to the Divine, to live Qonly for the Divine, to will only what the Divine wills, to express only the divine Will, to have no other source of energy than that of the Divine.
Ref: Questions and Answers (1953)