the religion which Christianity ruined in Rome was
Mithraism....with Ahura Mazda as a major figure.

It's true that the triumph of Christianity led to the demise of Mithraism, but this was not the Ahura Mazda religion (which was confined to Persia and points East), but a religion with the same name that developed much later (but before Christianity) in the Roman Empire. This religion revered Mithras, the Lord of the Universe, famous for his cloak of stars. It was particularly popular among the soldiers of the Empire, and its temples have been found in most places where the Romans had colonies, including London. A disadvantage (?) was that membership was restricted to males. Christianity copied several of its early tenets from this Mithraism, including the birthday of 25 December, the order of priests, baptism, parts of the sacrifice of the mass, etc.

Rest assured that we are not pulling anyone's leg about this. The idea of leaving the topic of God to be discovered by the individual himself, as mentioned above by Amos in relation to Scientology, is something that I must discuss with my friend shortly.