Daniel prophesized about a “Son of Man” — the term by which Jesus most often referred to himself, saying:
There came one like a son of man, and he came to the Ancient of Days and was presented before him. And to him was given dominion and glory and a kingdom, that all peoples, nations, and languages should serve him; his dominion is an everlasting dominion, which shall not pass away, and his kingdom one that shall not be destroyed.
They believe that these prophesies refer to a Christ who is actually divine. And because the Bible is clear that there is only one divine God, Christians conclude that this promised Christ must be God himself.
This is the Christian doctrine of the Trinity: that there is one all-knowing, all-present, and all-powerful God, who exists simultaneously in three separate and distinct persons: Father, Son, and Holy Spirit. Christians believe that 2,000 years ago the second person of the Godhead, God the Son, took on a human form without diminishing his divinity.
What Christians celebrate on December 25 is the miracle of the Incarnation, the idea of an infinite and eternal spiritual being who created the entire cosmos taking the form of a frail human child, born into a family of nobodies, in a backwater country, on a speck-of-dust of a planet.