Zoroastrianism is the name of the religion itself — the one founded by Zoroaster in ancient Persia, now Iran, and one of the oldest religions in the world still practised today. Its followers believe in one supreme good god, Ahura Mazda, and see the world as a long struggle between the forces of good and light and the forces of evil and darkness. Good is expected to win in the end, but not on its own: people must help it, and the way they help is beautifully simple to state and hard to do — good thoughts, good words, good deeds. Fire burns in their temples as a sign of purity and of God's presence, which is why outsiders have long called it fire-worship; that name is a bit unfair, and followers will tell you the fire is a direction to face, not the thing they pray to.

It was once the state religion of a huge empire. After the Arab conquest of Persia many followers left for India, where their descendants are the Parsees; today the whole community worldwide is small, perhaps well under two hundred thousand people. Historians often point out how many of its ideas — heaven and hell, a final judgement, a coming saviour — appear later in other religions, though how much was borrowed and how much grew up separately is argued about.

Capital Z, uncountable, no 'the': 'he studies Zoroastrianism', not 'the Zoroastrianism'. A formal word for a formal subject.