Sincerity lies in all distinguished actions: it is a notion that starts with acceptance and ends with Allah's pleasure. Therefore he whose actions Allah accepts and with whom He is content is the sincere one, even if his actions are few. Whoever does not have his actions accepted is not sincere, even if his actions are many, as we can see when we consider what happened with Adam and Iblis, may he be cursed.
The sign of acceptance is the existence of integrity and correctness, by expending all that is desirable with accurate awareness of every movement and stillness. In upholding what he has, the self of the sincere one is consumed, and his life is spent so as to put what he has in order, unifying knowledge and action, the doer and what is done by the action. For if he has attained that, he has attained all, and if he misses it he misses all; and that is brought about by purifying the meanings of disassociation (tanzih) in His unity.
As the first Imam said, 'Those who act will perish, except for those who worship; those who worship will perish except for those who know; those who know will perish except for those who are truthful; those who are truthful will perish except for those are sincere; those who are sincere will perish except for those who have precaution; those who have precaution will perish except for those who have certainty, and those who have certainty are of exalted character.' As Allah said,
وَاعْبُدْ رَبَّكَ حَتَّى يَأْتِيَكَ الْيَقِينُ
The lowest level of sincerity is when the bondsman exerts himself as much as he can, and then does not consider his action to have any worth with Allah so that he does not make his Lord recompense him for his actions according to his knowledge, for if Allah asks him to fulfill the full duties of slave hood ('ubudiyah) he would be unable to do so. The lowest station of the sincere person in the world is safety from all wrong actions, to be rescued from the Fire and to win the Garden in the next world.