Jesus of Nazareth is an important religious figure. Both Christianity and Islam accept him as the Messiah promised by the Old Testament, though other beliefs surrounding him differ.
Jesus in Christianity
Jesus is the central figure of Christianity. In Christianity, Jesus is seen as a direct revelation of God, or (usually) is viewed as God himself. Thinking of Jesus as God does not interfere with the Christian understanding of monotheism, due to Christian concepts of the Trinity.
According to the Bible, he was crucified by the Roman government at the urging of the dominant Jewish religious leaders of the time when he was 33 years old, but rose from the dead three days later. The belief that Jesus is the Christ (firstborn son of God, eternal King, etc.) and was raised from the dead, (thus alive and in power) is the basis of Salvation. Romans 10:9
One central Christian doctrine (for most branches) is that Jesus Christ has two natures: humanity and divinity. In many icons of the Eastern Orthodox Church this is given a visual portrayal: the figure of Christ is clothed in two opposing colours, red and blue. The doctrine of the two natures, that Christ is at once both fully human and fully divine, became widely accepted from the end of the fourth century, following numerous controversies and debates within the patristic world, though it was not always as universal as it is today.
In Christianity, it is in the person of Jesus Christ that the nature of God is revealed. For this reason Christians sometimes speak of "knowing God" through Christ. In Christianity, God is not something that humans are left to puzzle over and come to a systematic understanding of through philosophy or science; God chooses, in Christ, to reveal himself to the human race. This concept has been central to Christianity from earlier than the second century, with letters such as 2 Clement affirming that "we must think of Jesus Christ as of God."
Jesus as the Son of God
One of the fundamental beliefs of most Christian branches is that Jesus is the Son of God. Most agree that Jesus is God (the Father), but that he also became a man when he was born to the Virgin MaryLuke 1:30 (around the year 0, though the exact year is debated by historians).
As the Son of God, Jesus is also, in John's gospel, often referred to as the "Word" or Living Word of God, and "Immanuel" meaning "God with us". In the opening verses of the gospel, "The word became flesh and dwelt among us." John 1:14 The title "Son of God" emphasises the doctrine that Jesus was fully divine.
Jesus as the Son of Man
As the Son of Man, or "seed of the Woman" (Eve), he was ordained in the beginning (Genesis) to be the supreme judge of both men and angels and succeeded in defeating the rebellious fallen angel, Lucifer (Satan) and restoring Earth to its rightful authority which was lost by the first man, Adam through lawlessness (sin). Genesis 3 The title "Son of Man" emphasises the doctrine that Jesus was fully human.
Jesus as the Christ
The word "Christ" derives from the Greek (Christos) translation of the Hebrew word "mashiah" - anglicized this will be more familiar as "Messiah". The word "mashiah" means "one who has been anointed"; the tradition of the Old Testament was to anoint (cover with olive oil) priests, prophets and, in particular, kings. Kings were regarded as being appointed by God, and therefore the idea of an anointed one is bound to the theme of being chosen by God. This also contains the allusion to royalty which became an important issue for Jesus during his life.
Jesus in Islam
In Islam, Jesus (who is the "Christ") is known as Isa Al-Masih which is Arabic for "Jesus the Messiah." He is viewed as an important prophet, but not a divine one. He is both a Prophet as well as a Messenger (Rasul), and is given particular respect, but in most versions of Islam, God is perfectly singular, and there is no space at all for any incarnation of God. In Islam, even those claiming to be in perfect union with God may be viewed as blasphemous (as was the case with the Muslim mystic al-Hallaj when he said "I am the Truth", for which he was crucified for blasphemy).
The Islamic understanding of Jesus is that he was a prophet of the same standing of Moses and Muhammed, and that rather than suffering crucifixion he was taken up by God at the moment before his death. While the New Testament of Christianity claims that Jesus rose from the dead, Islam asserts that this could not have occured; he did not die on the cross in the first place.