"Mithrism" was a mystery religion prevalent among the Roman Legions - the name giving it an aura of esoteric authority. It was a "New Age" religion keyed to the precession of the equinox, similar to the "Age of Aquarius", but a previous transition. The sect was very secretive so it is not known just how much was adopted by Christianity.

Christianity was also definitely a "New Age" religion - until the Old Testament got tacked on to it to justify and authoritarian Church.

Personally, I don't think the Old Testament has any business being in Christianity, and some Christian sects do agree.