A summary of the concepts held by the Ahl al-Sunnah.
The difference between truthfulness (sidq) and sincerity (ikhlas).
The difference between truthfulness and sincerity in the realm of creed.
Truthfulness is the opposite of the complete absence of the intention to seek the countenance of Allah through one's deeds.
Such as one who believes or prays while being a liar.
This is hypocrisy of the heart.
As for sincerity, its opposite is the failure to single out Allah alone in one's intention and orientation.
Such as one who believes and prays while directing that toward someone else alongside Allah.
Among the ways to distinguish between truthfulness and sincerity is also the fact that
Truthfulness is not restricted to creed alone.
Rather, it also pertains to speech—which is the most common usage—and to actions.
Therefore, in proportion to a person’s realization of the branches of faith and its deeds,
Is their share of truthfulness, until they reach the rank of the truthful (siddiqiyyah).
As for sincerity, it is a purely inward act of the heart.
Only its effects manifest upon the limbs.
Despite this,
It is possible, with some latitude and flexibility,
To use one term or its opposite in place of the other or its opposite.
Thus, it may be said of the hypocrite that he is not sincere to Allah.
Just as it may be said of the polytheist that he is untruthful in his faith.