Mithra — like Tammuz, Adonis, Apollo and Horus — was a sun-god. So you see, the "reason for the season" is not the son of any god, but the sun up in the sky!
"Religion," Rand noted, "is the first enemy of the ability to think. ...yet before they learn to think [men] are discouraged by being ordered to take things on faith. Faith is the worst curse of mankind."