Sincerity is an attitude of the heart and God considers a man according to the inclinations of his heart. Rather, He considers your hearts . Sincerity is such a mysterious Divine credit granted to pure-hearted ones which increases what is little and deepens what is shallow and which gives infinitude to finite worship, that a man can seek to purchase with it the most valuable things in the markets of this world and the next, and is esteemed and receives welcome and respect where others suffer great miseries. If a deed may be considered as a body, sincerity is its soul, or if a deed may be considered as one of the pair of wings to fly with, sincerity is the other. A body without soul is of no worth, and it is impossible to fly with a single wing.
Sincerity is the wing of the bird of the acts of obedience.
The treasures of God Almighty are full of acts of worship. If you intend to reach Him, see yourself as small at the door of God and be sincere in your deeds. For some, sincerity is refraining from being seen by others while doing supererogatory deeds and avoiding show and ostentation. For some others, it is that while doing religious deeds one should not consider whether others see one or not. Still for others, it is engrossment in worship or religious deeds in consideration of God's good pleasure to the extent that one does not even remember whether one should be sincere or not.