The Man Jesus is what the people of Mid-World called the son of God, which is similar to that of our world's Jesus.
He is mentioned many times in the Dark Tower series as either "Man Jesus" or "The Man Jesus". It is also mentioned that he was able to perform miracles and was of higher descent which is why it is assumed he is like the Jesus of our world.
In her sermons, Sylvia Pittston preached of the Man Jesus and an Anti-Christ figure called The Interloper.
In Wizard and Glass, Eldred Jonas plays a card game called Chancellors where there are twelve Chancellor cards modeled after Jesus's apostles.