December 25th is not Jesus Christ’s birth day which is clear from bible story of Jesus Christ’s birth, rather it was adopted from ancient pagan Roman culture as the birth day of sun or mithra. At council of Nicea at 325 Christians adopted another pagan concept that is trinity but even at that council Holy Spirit was not declared as God, only elevated the status of Jesus equivalent to God. Sometime in 430s Holy Spirit was elevated to the status of God. However all these confusions started as different bible writer perceived Jesus Christ differently. For example Paul perceived Jesus as someone who is above angel but below the status of father who is also god not God through whom father does everything and maintain connection between human being and father. In other words it was a gnostic view which believes God does everything via lesser gods. Other book only perceived Jesus as a good teacher and human being not God, for example gospel of Mark, Matthew and Luke, Book of James which is supposedly written by brother of Jesus Christ. In 2nd century Christian belief was that Jesus Christ died and rose again after 3rd day and ascendent to heaven and seat beside the right hand of God. In third century it created a theological problem which eventually created Arian controversy. To Arian Jesus Christ can not be God or god as he sat right hand of God which makes Christianity a paganism. If there is a father and son relationship that means Jesus Christ is creation not creator. Other group saw it as another theoretical problem. If Jesus Christ is not creator God how he forgave every one’s sin by dying at the cross as a result it became necessary to elevate the status of Jesus Christ to God tried to give the assumption that Jesus and Father are same thing and they were there from eternity and later in 5th century Holy Spirit was elevated to the status of God as well. Upto third century most of the Church fathers except few even believed Jesus is God. Gnostic view of Jesus was popular. I hope you got the idea how Jesus Christ is son of God this idea has become popular.