Mithras, not to be confused with Mitra (the warrior angel of ancient Persia), was the head deity of Mithraism. Trying to piece together the actual legends relating to Mithras is difficult as the earliest evidence relating to him is only found in artistic reliefs- the original texts regarding Mithraism have long since been lost, leaving behind only fragments. For this discussion, we will focus on Roman Mithraism as this is the Mithras the critics claim as being the inspiration for Jesus (although this allegation could easily be dismissed by showing most texts containing the alleged connections postdate the Christian texts). Furthermore, Roman Mithraism surfaced centuries after the existence of the Hebrew Messianic prophecies.
Note: The original authority on Mithraism was Franz Cumont who believed the Mitra of ancient Persian and the Mithras of Mithraism were one and the same. Most of his research was compiled in the 1800's and, because he was the first known scholar to explore the dead religion of Mithraism, his research went undisputed for quite awhile. If you look through early 20th century publications, one can see Cumont's findings were accepted without debate. It was only upon later investigation by differing historians and archaeologists that many of Cumont's theories were disproved. To see what I am referring to, read this article from the 1911 Encyclopedia Britannica which is based on Cumont's theories as compared to more modern research.
CAVE BIRTH As stated previously, there is no mention of Jesus being born in a cave anywhere in the canonical Scriptures. As for Mithras, he also was not born in a cave but from solid rock.
DECEMBER BIRTH Many religious festivals were consolidated into one holiday to coincide with the winter solstice. Christmas is only celebrated on December 25th due to this tradition. This argument already proves to be insignificant as there is nothing in the Scriptures which mentions this date.
ATTENDED BY SHEPHERDS The earliest existing account of Mithras' birth is found in a relief depicting him emerging from a rock with the assistance of men who certainly appear to be shepherds (which is interesting considering his birth was supposed to have preceded the creation of humans!). But this little tidbit was added later, apparently by those who didn't notice the contradiction. Furthermore, this relief dates to 4th century A.D.!
VIRGIN BIRTH There is no mention of a virgin birth in Mithraism. The earliest reliefs depict a fully-mature Mithras emerging from a rock (as shown in three illustrations to the left).
TWELVE DISCIPLES Mithras did not have twelve disciples, but I can relate a far-fetched similarity to this allegation. In two of the reliefs to the left, Mithras is surrounded by the twelve signs of the zodiac. Claiming Mithras had twelve disciples because there are twelve signs of the zodiac is the connection critics try to make. The critics simply see twelve beings and claim the figures are disciples. Some go as far to defend their position by mimicking Franz Cumont's theory, claiming the figures were actually Mithras' twelve disciples dressed up in zodiac costumes! How they can make this connection is unknown as no inscriptions accompany the original reliefs.
GREAT TEACHER I can find no mention in any text or relief showing Mithras to be a traveling teacher. Regardless, it would hardly seem significant as many legends speak of mankind receiving wisdom from their gods.
ATONEMENT OF SIN The claim regarding Mithras atoning for sin leads me to as the question, how? There is no mention of this in any record. Mithras does sacrifice a sacred bull to create life but I see no reference to the atoning of sin, the atoning of sin through blood, or Mithras atoning for sin. Some try to merge the bull and Mithras into one being but this concept is unanimously rejected by Mithras scholars.
LAST SUPPER There are two reliefs which show Mithras celebrating a banquet. The first relief shows Mithras and Helios dining together after the sacrifice of the bull. The other depicts Mithras dining with the sun before ascending into paradise with the other gods. But for some reason the tale becomes distorted with Mithras saying to his (imaginary) disciples, "He who shall not eat of my body nor drink of my blood so that he may be one with me and I with him, shall not be saved." Yet this quote was added centuries later during the middle ages and is not even attributed to Mithras!
CRUCIFIXION Though critics claim Mithras was crucified, there is no mention of this in the reliefs or texts. In fact no death is associated with Mithras, nevertheless crucifixion. We are told he completes his earthly mission then is taken to paradise in a chariot- alive and well.
SUNDAY AS A HOLY DAY This appears to be correct, at least for Roman Mithraism. But considering almost every religion used Saturday or Sunday as a holy day, there was a 50/50 chance of this hitting the target- or at least a 1/7 chance for the number of days in a week. Christians selected Sunday as their holy day only because it was the day of Christ's resurrection.
SIMILAR TITLES I did find some similarities but the claims critics make seem to be manipulated from their original form- there were no exact matches to the names critics list. I also listed other titles that are often cited but prove to be incorrect:
Savior, Redeemer, Messiah. Mithras is never referred to any of these. Why would he be since he never served such a purpose? Messiah is also a Hebrew word which makes one wonder what the source is for this allegation.
Lamb of God, Good Shepherd. Skeptics try to use the depiction of Mithras holding the sacrificed bull over his shoulders as evidence but this is absurd as the bull is slaughtered! Furthermore, the Old Testament references lambs and shepherds long before Mithraism ever surfaced.
Son of God. I didn't technically find this but I'll give it as a freebie if you consider Mithras as the son of Ahura Mazda.
The Way Truth and Light, Light of the World. Though the names are not an exact match I did find warrior angel of light but this is associated with the Iranian Mithras- not the Roman Mithras of Mithraism.
Lion. Again, not an exact match but I did find an association to sky/celestial lion, referring to the sign of Leo. But like the reference to lambs, the Old Testament mentions the Lion of Judah long before Mithraism ever originated.
The Living Word. Mithras is sometimes called logos which means word but never as the living word.
Mediator. Mithras was the mediator between good and evil whereas Jesus is the mediator between God and man.
THEOLOGICAL COMPARISONS I consolidated the similarities that come standard in most religions into one section. For lack of a better title, we'll call this our stating the obvious list:
Mithraism had a strong sense of community among its members (only men were allowed to be members, by the way)
Mithraism taught the immortality of the human soul (so did Judaism which preceded Mithraism)
Mithraism placed emphasis on living an ethical and moral life (so did Judaism which preceded Mithraism)
Mithraism believed in the concept of good verses evil (so did Judaism which preceded Mithraism)
Mithraism taught all life sprouted from god(s) (so did Judaism which preceded Mithraism)
Mithras performed miraculous deeds
Mithraism taught the eventual destruction of the earth
ERRONEOUS SIMILARITIES The following miscellaneous similarities exist neither in the ancient reliefs of Mithras or in any version of the surviving texts:
Mithras began his ministry at the age of 30 (no reference to any age is mentioned).
Mithras was buried in a tomb (Alive, I suppose?). The only reference I could find to this was every year during the winter solstice, he was supposedly reborn out of a rock (but this tale was added later).
A Holy Trinity (Even with all sorts of new gods becoming associated with Mithraism over time, I can find no mention of any gods forming a specific trinity).
CONCLUSION Once again, the alleged similarities are either superficial, completely fabricated, or stretched to make a match.