What is the response to a person who claims to be a Muslim but doubts the truthfulness of God's promises, the Quran, and the Sunnah, claiming that divine promises are mere words and that the Sunnah was transmitted by people who might be liars, and what is the ruling on such a person in cases of public doubt or internal whispers?

Denying the promises and warnings mentioned in the two revelations (Quran and Sunnah) is considered disbelief (kufr) if the conditions for declaring a specific individual a disbeliever are met. As for the whispers of the heart concerning this, if the person disavows them in their heart, there is no blame upon them, because Allah has overlooked what Satan whispers as long as it is not spoken or acted upon. However, if the person is convinced in their heart by these satanic whispers, then their heart's conviction in disbelief indicates their disbelief.