He was a demi-diety, in that sense. However, he was also the God of the old Testiment. Christ was the one who created the world (under the direction of his Father). He is the one that spoke to the ancient prophets. He is a full god, equal to his Father in power, but under him in authority. So, while he had a mortal mother, he was always a God, and was never a half God, as the term Demi-Diety suggests.

The term demi-diety implies that God, or gods, are a separate race from humans. Because of this a person who had a mortal parent and a devine parent were half god, thus demi-gods, or demi-deities. In this sense, Christ was not a demi-diety. God and Christ, and all otehr gods that exist, are human, but ressuracted and glorified. Because of this no one can be a demi-god. You are either a God or you are not. Jesus was.

So, we do not believe in a demi-diety, though others may see it as that.