A sin from which we have not repented or that which we continue engaging in without any intention of giving up has many negative effects.
It makes us disinterested in benefiting from any religious or spiritual programs. It causes physical, emotional & mental ailments & stress. It disallows us from distinguishing right from wrong, good from bad, permissible from prohibited. It results in loss of enjoyment & concentration in prayer. It makes us befriend our enemies & consider our true friends as enemies. It finally causes arrogance that makes us think we do not need to leave the sin because the Almighty is Most Forgiving.
The Almighty says some become similar to those affected to the degree of insanity by the touch of the devil.
May we never be from those who cannot give up a sin!