Mithraism originated in Persia quite a bit before the Romans adopted it. While I agree that God talks to all people throughout history (and even to this day), it is also my belief that a goodly portion of that which is attributed to Jesus was simply lifted wholesale from other religions.
I disagree, Jesus would seem to me to have said most if not all of what is attributed to him. He would not have had the following he had otherwise and it is clear in life many were listening.
I will agree that the Christian religion has picked up Holidays, traditions, etc from other religions but I don't doubt Jesus said the things it is reported he said.