When the Christ myth was new Mithras and Mithraism were already ancient. Worshiped for centuries as God's Messenger of Truth, Mithras was long revered by the Persians (Zoroastrianism) and the Indians (see the Vedic literature).
Dating Mithras in Rome:
Plutarch (Pompey, 24, 7) and Servilius (Georgics, 4, 127) say Pompey imported Mithraism into Rome after defeating the Cilician pirates around 70 BCEE. Mithras appears epigraphically in the circles of the Roman emperor in the first century CE―around the time the canonical Christian Gospels were written (Corpus Incscriptionum Latinarum, 6, 732), and statues of the God were present by 101 CE (Corpus Incscriptionum Latinarum, 6, 718). As with Attis, Christian apologist Justin (1 Apologia, 66, 4) denounces the devil for having sent a God so similar to Jesus -- yet preceding him. *
Surely there's a lot we don't know about this faith that comforted million of souls. Early Christians established the dominance of their religion by exterminating Mithras' faithful, razing His temples, burning His sacred texts. HOWEVER,
We do know this:
With twelve disciples he traveled far and wide as a teacher and illuminator of men.
He was buried in a tomb from which he rose again from the dead―an event celebrated yearly with much rejoicing.
Every year in Rome, in the middle of winter, the Son of God was born one more, putting an end to darkness.
Every year at first minute of December 25th the temple of Mithras was lit with candles, priests in white garments celebrated the birth of the Son of God and boys burned incense.
Mithras was born in a cave, on December 25th, of a virgin mother. He came from heaven to be born as a man, to redeem men from their sin.
He was known as "Savior," "Son of God," "Redeemer," and "Lamb of God."
His followers kept the Sabbath holy, eating sacramental meals in remembrance of Him. The sacred meal of bread and water, or bread and wine, was symbolic of the body and blood of the sacred bull.
Baptism in the blood of the bull (taurobolum)―early Baptism "washed in the blood of the Lamb"―late Baptism by water [recorded by the Christian author Tertullian] Mithraic rituals brought about the transformation and Salvation of His adherents―an ascent of the soul of the adherent into the realm of the divine. From the wall of a Mithraic temple in Rome: "And thou hast saved us by shedding the eternal blood."
The great Mithraic festivals celebrated his birth (at the winter solstice) and his death and resurrection (at the spring solstice)
* Pagan Origins of the Christian Myth, Dating Mithras, [ http://www.entheology.org/POCM/pagan_christs_mithras.html ]