A footnote in James Still's "Critique of New Testament Reliability and 'Bias' in NT Development" reads:
". . . Justin obtained some stories which are no longer a part of the canon or any known oral tradition. He relates that Jesus was born in a cave and that a fire erupted in the river Jordan upon Jesus' baptism. (Dialogue with Trypho the Jew, 78 and 88). Curiously, the pagan god Mithras was also said to have been born in a cave while shepherds kept watch."First, Mithras/Mitra (originally a lessor deity in the proto-Hindu and Zoroastrian pantheons) is such a pan-religious figure that just about EVERYTHING has been said about him! So what?!
Second, ALL of our data about the Mithraic cosmogony (including his 'birth') comes from art works--there is NO primary textual data whatsoever to work with.
Third, by the time we DO get some textual data, it shows up in varied and contradictory snapshots--from the various religions--and it DOESN'T support this claim.
Fourth, Mithras was portrayed in art works as having been made out of rock, and then CREATING the world-cave (MM:203-207; FRC:276-279)--NOT being originally born 'in a cave'.
Fifth, the passage in Justin about Mithra (assuming Jim is taking his cue from JM's remarks) is NOT talking about Mithra being BORN in a cave, but in 'borning' his initiates (i.e. converts) in the 'caves' in which they worshipped. The Mithraic cult, esp. in the 2nd century, met in underground caves (or tunnels made to resemble caves). The 'cave' motif was in honor of (a) Mithra's creation of the world-cave and (b) Mithra's slaying of the 'bull' in a cave.
Finally, ONE of the many splinter groups of Mithraism (the Armenian group) imposed a season-fertility motif on Mithra, and had him yearly go into a cave (winter) and reappear (be 'reborn') each spring. This can hardly be called being originally 'born in a cave'!
This topic is exceedingly complex, and much of the work of the early leader in the field (i.e. Cumont) has been overturned in the last 20 years (including some of the popular 'myths' thrown around on the Internet!). [For a balanced overview and biblio on Mithraic issues, see the Ecole Initiative on the Web.]