Why are Christians and Jews tormented, and what is their sin?

"Waswasah" (whisperings/obsessive thoughts) are of two types: mere fleeting thoughts that disappear, and their cure is to divert one's mind from them; and thoughts that develop into ideas that settle in the mind, and their cure is through knowledge, evidence, and presenting them to trustworthy scholars. You must be certain of Allah's absolute justice and that He wrongs not even an atom's weight, and that shirk (associating partners with Allah) is the reason for the punishment of the Jews and Christians. Among the causes for treating waswasah are: abundant remembrance of Allah (dhikr), seeking forgiveness (istighfar), reading the Quran—especially the two Mu'awwidhat (Surahs Al-Falaq and An-Nas), Surah Al-Ikhlas, and Surah Al-Baqarah—engaging in seeking religious knowledge, associating with righteous people and scholars, and supplicating to Allah with earnest humility.