Jesus Christ is revered by Christians as the Son of God and come Dec. 25 (and a little later for some other denominations) believers around the world will celebrate the founder and namesake of the world's largest religion. During his time on Earth, however, Jesus' followed a much older religion and based his beliefs on its teachings.
Jesus was a Jew. While not much is known about Jesus' adolescent years, it is believed that he regularly attended Temple and showed an almost supernatural knowledge of his Jewish faith. In one Bible story, a 12-year-old Jesus is separated from his parents and found three days later discussing Judaism with religious teachers who were stunned by the young boy's extraordinary knowledge.
As he grew up and began to preach, he did not distinguish his faith from Judaism, which dates back to the Bronze Age, however, his claim to be the Son of God disturbed many at the time. Jews believe that they are the chosen people of God and that one day he would send a Messiah to reunite the ancient tribes of Israel, rebuild the Temple of Solomon and usher in a new, wondrous era known as the Messianic Age. Most Jews rejected Jesus' claim that he was the Messiah and especially denied that he was the Son of God. Those that believed him, became Christians.
One of the early, main doctrinal differences between Judaism and Christianity was the emphasis the latter placed on Jesus and the Holy Spirit, two extensions of God. Judaism has a strong religious tradition that stresses God could not share any mortal attributes, so it could not accept Jesus as the Son of God who was both human and divine. Jesus focused less on the Jewish holy book, the Torah, and more on personal understanding of God, according to religious tradition. Because he spoke of the "Kingdom of God," Jesus was sometimes referred to as "King of the Jews," a charge that eventually saw him found guilty of sedition and crucified by Rome.
Most Jews today do not accept that Jesus was the Son of God, beyond that, there is no official Jewish teaching on the man who professed to be the Messiah. There are a small number of Jewish sects, such as Messianic Judaism, that do believe Jesus to be the Messiah and the Son of God, but retain their Jewish identity distinct from Christianity.
Long after his death, Jesus became an important figure in a third Abrahamic religion, Islam. Muslims revere Jesus as one of their holiest prophets and, like Christians, believe he was conceived miraculously by his mother Mary through God's intervention. Islam also states that Jesus was crucified, but rejects the notion that he was killed on the cross. Instead, they believe Jesus was entirely mortal, not divine, and God accepted Jesus into heaven before he was killed, only making it seem as if he was crucified. Muslims believe Jesus will return at the end of the world to fight alongside the Mehdi, a Muslim leader, to defeat Satan and the Anti-Christ.