2. Mithraism
Mithraism thrived within the Roman Empire, especially among soldiers from the 1st to 4th centuries CE. Centered on the worship of the god Mithras, followers participated in mysterious rituals conducted in underground temples known as Mithraea. Its secretive nature and exclusive membership made it influential across the empire. However, as Christianity rose to prominence, Mithraism quickly declined and was eventually forgotten. History.com on Mithraism
