In Justin Martyr’s First Apology to the Roman emperor, Marcus Aurelius, Justin gives a very detailed account of the Eucharist. I personally believe that Martyr’s description is the clearest early proof of the Real Presence (The gospels, Paul and Ignatius clearly demonstrate the Real Presence in their writings–but they don’t address it as directly as does Justin.)
The disturbing part of Martyr’s description of the Eucharist comes in the last two lines. He mentions in these two lines Mithraism and how its adherents are stealing the idea of the Eucharist and using it in their own ceremonies. While I believe that it is more likely that the Mithraists are indeed stealing (or perhaps only coincidentally share) a Euchuaristic-like ceremony, there is also the possibility that early Christianity merged with Mithraism to create the concept of the Eucharist.
How do we know that the early Christians received their Eucharist from the apostles and from Jesus and not from some Mithraic religion? Better yet, why did the Mithraic adherents have a ceremony similiar to the Christian Eucharist?
The following is a quote from Martyr’s First Apology and is to be found at www.newadvent.org:
For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, “This do ye in remembrance of Me, this is My body;” and that, after the same manner, having taken the cup and given thanks, He said, “This is My blood;” and gave it to them alone. *Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn. *