What should a person do who has, in his heart, considered a sin lawful, thereby falling into disbelief; is there any repentance and salvation for him; and what should he do if heedlessness and hardness of heart afflict him, knowing that he was a believer and devout before committing this transgression?

The istihlal (considering something permissible that is forbidden) mentioned, if it is merely a fleeting thought and a whisper (waswasa), then it does no harm. This is because Allah has forgiven this Ummah for whatever they think to themselves, as long as they do not act upon it or speak of it. If, however, the heart inclines towards istihlal, then one must repent and feel remorse, and firmly resolve in the heart to consider forbidden what one had believed permissible. He who repents from a sin is like him who has no sin. As for hardness of heart, its remedy lies in turning to Allah sincerely and supplicating Him. The questioner is warned against giving in to whispers.