Mithraism was a mystery religion (a religion requiring an initiation rite and secrecy from non-members) popular in the Roman Empire from the first to the fourth century AD. The members met in a cave or other underground structure, known as a mithraeum, and we have found hundreds of them all over the Roman Empire (modern Italy, Spain, Germany, the United Kingdom, Greece, Algeria, Israel, and many other countries). The central focus of the mithraeums are an image Mithras killing a bull, accompanied with a dog, a snake, a scorpion, and a raven. These characters are all zodiac symbols.
Is Mithras the origin of the Christ story? Jesus and Mithras do share some properties, but that’s not exactly what comparative mythology says. Comparative mythology merely points out that deities and supernatural beings before/after/during the rise of Christianity have many similar properties. Perhaps some of the concepts are basic human tendencies, like wanting to “defeat” death. Perhaps others are cultural, and many appear to be astronomical. There will be many theories, but unlike scientific theories, there is no real way to test them for validity. They can only be analyzed for probability.
Christians wanting to deny any comparisons to Christianity have claimed that Mithras was not early enough to be a source. However, we have learned from Plutarch that Mithraism was widely practiced in 67BC by Cilician pirates, putting it’s probable origin before that.
The virgin birth of Mithras was celebrated on December 25th, the same as many other mythological characters both before and after the origin of the Jesus story. The celebration of Christmas only goes back to the 2nd century, but it was not observed on December 25th until Pope Julius the 1st picked that date in 350 AD. And, even though Christian tradition today says that Jesus was born of a virgin, this does not appear to be an original aspect of the Jesus story. The earliest accounts of Jesus (the epistles of Paul are commonly dated to 50 AD and the first gospel, Mark, is commonly dated to 70 AD) do not mention a virgin birth at all. The addition of a virgin birth aspect to the Jesus story may be the result of an observations of religions called syncretism, which is the tendency of religions to adopt additional beliefs over time.
Origins of the Mithraic Mysteries by David Ulansey (recommended book)