What is the ruling on obsessive extreme fear of Christians, Shiites, and Baha'is, and of ideas related to them and to other matters in religion?

The cure for obsessive thoughts (waswasah) is to completely disregard them and not dwell on them. Indeed, ignoring them will make them disappear after a short while. However, listening to them will only increase them until they lead to madness. It is also recommended to frequently recite the prescribed invocations and supplications for protection, such as the prayer for pardon and well-being (du'a al-afw wal-'afiyah), and to recite Surat al-Ikhlas (definition) and the two Mu'awwidhat (Surat al-Falaq and Surat an-Nas) three times in the morning and evening, and to say "Hasbiyallahu la ilaha illa Huwa 'alayhi tawakkaltu wa Huwa Rabbu al-'Arshil-'Azeem" (Allah is sufficient for me; there is no deity except Him. In Him I have put my trust, and He is the Lord of the Mighty Throne) seven times in the morning and evening.