What is the ruling on my saying "Alhamdulillah" to someone who described me as "the holy man"—meaning a Christian clergyman—even though I did not intend to agree with his description, but rather to answer his question about my well-being? And is someone excused who forgets a religious ruling that he once knew?

If you answer someone who calls you by a word of disbelief (kufr), and your intention is to respond to their question about your well-being, not to agree with their calling you by that term, then that is not considered disbelief. Al-Nawawi and Ibn Hajar al-Haytami have stated that ruling someone to be a disbeliever in such a case is questionable, unless there was an intention to be pleased with what was attributed to him. The same applies to the issue of imitating disbelievers; one does not disbelieve by doing so unless their intention is to be pleased with their religion, or to incline towards it, or to belittle Islam.