When is belittling a sin considered disbelief (kufr), and when is it not?

The Sunnis are in agreement that a person who commits a sin does not become an unbeliever unless he deems it permissible (istiḥlāl). And belittling a sin, which is daring to commit it and being deluded by forgiveness, does not necessarily lead to disbelief, unless it is a form of deeming it permissible that necessitates exiting the fold of Islam.