The symbolic and ritualistic similarities between Mithraism and Christianity go beyond coincidence - indeed, many aspects of the two religions are clearly almost identical. Consider:
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Mithras was born on December 25th (the winter solstice, also the date of the Roman natalis solis invicti, or "birthday of the unconquered sun") while simple shepherds stood by in wonder. His birthplace was a cave, as was that of the early church's version of Christ. Mithras was often referred to as "the good shepherd" and depicted carrying a bull across his shoulders.
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Tertullian writes that followers of Mithras practiced baptism by water, a ritual that was supposed to cleanse them of sin, while the priest made a sign upon their foreheads. They also consumed a sacred meal of bread and wine, said to represent the flesh and blood of the bull aspect of the god; Sunday was a sabbath for them.
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In one story, Mithras strikes a rock and calls forth water for his followers. In another, he participates in a final feast before ascending to heaven.
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One tenet of Mithraism was that there would be a "judgement day" when unbelievers would perish and the faithful would live forever with Mithras in paradise. Mithraism recognized no social distinctions - all that was required for one to be saved was to follow Mithras the dominus (lord.) A good Mithraist practiced mortification of the flesh and repudiation of the world; part of the initiation rites was the rejection of an earthly crown and the declaration that the initiate looked to a heavenly crown instead.
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Mithraists were called "Soldiers of Mithras," and considered themselves duty-bound to "put on the armor of light" (Mithras being the god of light) and struggle against evil.
This is not to suggest that Christianity is a modified form of Mithraism. Instead, it serves to show how adept the early church leadership was at appropriating any ritual or symbol necessary to make Christianity appeal to the masses - for in the beginning, Christianity was just one of many mystery religions competing for the hearts and minds of the Empire. It succeeded where the others (the cult of Isis, for example) - failed and disappeared, because it constantly changed, absorbing the tenets (and thus the followers) of its competitors. Mithraism was particularly prevalant among the ranks of the Roman army, a group whose loyalty was obviously key to the survival of any organization - what better way to make your own religion acceptable to such people than to clothe it in familiar trappings?