Mithraism was a religion that was important during the Roman imperial period. It was one of the "mystery religions" in which devotees believed they had secret knowledge that would have benefits like a good afterlife that were not really part of the official Roman religion. Mithraism was perhaps the most important of these mystery religions.
Mithraism was an offshoot of Persian religion. Mithra was a Persian god associated with the sun. In Roman times, he was called the "unconquered sun" and worship of him was for men only. This was a very popular religion among soldiers.
Mithraism has striking similarities to Christianity in that both worshipped on Sunday, both celebrated the birth of their god around December 25, and both celebrated ceremonial meals together. These similarities lead some to argue that Christianity borrowed ideas from Mithraism, but there is no solid evidence that this happened.