OK, here are a few facts about Mithraism.
First, scholars are very divided about what exactly Mithraism is and what counts as "Mithraist" cult. It's like Gnosticism - the definition is hard to refine precisely. However, the consensus today is that Mithraism *as it is usually defined* came into being probably in the early second century AD. One of the key elements of Mithraism is that it took place in mithraea, that is, the underground temples. Being underground, these are typically well preserved. But the earliest one we know about is probably from the middle of the second century.
Thus, it seems that Mithraism came into being some half a century after Christianity. Of course, Mithras himself was a much older god than that, having been worshipped in Persia for God knows how long. Similarly, the Tauroctony - the image of Mithras killing a bull, which is *the* iconic image of Mithraism - seems to have emerged something like a century before Christ, I think. But still, the various elements of what we know as Mithraism seem to have come together in the second century AD, or not long before. Mithraism per se is thus a religion of the Roman empire, based upon the worship of a Persian god - Mithras was Persian, but Mithraism was not (I suppose it's rather like Mormonism being an American religion that worships the Middle Eastern Jesus!).
We don't know much about what Mithraists believed. Basically, we have almost uniformly brief and hostile reports in later Christian writers, and we have the actual sculptures and reliefs in the mithraea. It's hard to reconstruct a religion on the basis of these. But a few points are in order:
Mithras is never represented as born of any person at all, virgin or not. He is typically represented as emerging from a cave or a rock - hence the name Mithras Petragenetrix, or Mithras Rock-Born.
Mithras is also never represented as dying, rising again, or anything like that. But he is represented as rising to heaven, to sit at the right hand of the sun.
Contrary to popular belief, Mithras wasn't said to have been born on 25 December. 25 December was the date of the festival of Sol Invictis. But this was an entirely different cult, and in any case, the date was established by imperial decree in the third century (long after both Mithraism and Christianity had appeared). I think the confusion comes from the fact that later writers often claimed that Mithras was believed to be a sun god, but the evidence of the mithraea would seem to go against this, since Mithras and the sun are depicted as distinct entities. Plus, of course, 25 December is not the winter solstice. That was 21 December or 14 December, depending on which calendar you're using.
We don't know when or how often Mithraic rituals took place, whether on Sundays or not.
Rambuchan, where did you get the information on the Mithraist priests' outfits and titles? I have never heard anything like that, I must say.
Mithraism did feature cultic meals of bread and wine, although it's worth pointing out that pretty much every religion of this period did.
Just to pre-empt anyone who posts some long list of supposed similarities between Jesus and Mithras, it's also worth saying that Mithras is never represented as preaching, doing miracles, having twelve disciples, being a shepherd, being a sacrifice, being a redeemer, or pretty much anything else that Jesus is said to have done! And even if he were, remember that Mithraism appeared *after* Christianity did, so were there to be any influence, it would be more likely to go from Christianity to Mithraism, not vice versa. Certainly most of the cultic elements of Christianity, and I think most of its distinctive doctrines too, can be more easily traced to Judaism than to the mystery religions.
Besides all this, as I mentioned in the other thread, Mithraism and Christianity were quite dissimilar sociologically. Mithraism was like a secret society (it was one of the mystery religions, after all), open only to men, popular among the more elite classes. Christianity, however, was much more open, and was especially popular among the lower classes, women, and slaves. So they weren't even competitors in the same market, as it were.
As for the historicity of Mithras and Jesus, I'd say there's no comparison, really. Mithras is obviously a mythical figure. The images in the mithraea portray him doing acts of cosmic significance - emerging from a rock, killing some kind of cosmic bull, ascending to the sun, and so on. His Persian roots are represented in his outfit, which always features a sort of Buster Keaton style hat supposed to be Persian. Mithras is a timeless figure. Jesus, by contrast, was believed to have been a real historical person who actually lived relatively recently and really did things in a historical location. And indeed, there is plenty of evidence for who Jesus was and what he did. This was one of the key elements of Christianity that distinguished it from the mystery religions.
Finally, here are two very useful sites that should help to clear away all the rubbish one hears on this subject.
This site details pretty much all we know about Mithraism, primarily from the archaeological evidence. Note, in particular, the section called "What Mithraism isn't", which deals with the supposed similarities between Mithraism and Christianity. And
here is a page from an excellent site which lists all of the ancient literary references to Mithraism.
[EDIT] I've just been having a look at the Wikipedia article on Mithraism. It's got a big warning banner at the top saying that the information it contains is possibly partisan, and damned right too, since it is full of complete falsehoods! Pelagianism listed as a kind of Gnosticism?? Wikipedia is of course of variable reliability, but when it comes to subjects like this you just can't trust it, believe me.