What is the ruling on one who is assailed by thoughts of associating disbelief with a sin or a permissible act, and sometimes responds to these thoughts out of fear of disbelief? Is this considered disbelief, or is it merely from the whispers (waswas) that should be ignored?

These thoughts do not lead the person afflicted with whispers (waswas) to disbelief (kufr), especially if he detests and rejects them, and sometimes interacting with them does not result in accepting them. Therefore, one must divert their mind from these whispers and be certain that they do not constitute kufr.