The story
From Zarathustra + -ian + -ism, coined by Hermann Lommel and advanced by Ilya Gershevitch in the Hegelian dialectic view of Zoroastrianism in terms of thesis, antithesis, synthesis. In this construct, 'Zarathustrianism' is the thesis, 'Zarathustricism' is the antithesis, and 'Zoroastrianism' the synthesis.
Etymology from Wiktionary, CC BY-SA 4.0.