Christianity is a monotheist religion of Abrahamic descent, based loosely on the life, death, feats, adventures and teachings of Jesus of Nazareth and his league of super apostles, presented in the Christian New Bible or New Testament. However, Christianity also recognizes the Old Testament, such as Judaism and Islam. The difference is that Christians also recognize Jesus as the one true prophet who is a direct heir of God. In other words, they believe that Jesus is the true Messiah or the Christ, as prophesied in the Old Testament. Here is where they differ from Judaism and Islam. The former holds that the true Messiah has not arrived yet and Islam believes that he did much later and he was actually Muhammad. Although all these beliefs sound like sci-fi, these 3 religions are professed by billions of people around the world. By Christian standards, Jesus is supposed to have died, crucified to rid humans of their sins and wash them away, and resuscitated 3 days later.
The beginnings of this religion are traced back to the 1st century (Anno Domini), when rock-star Jesus of Nazareth is supposed to have walked the Earth (and water), gaining nicknames like J Waterwalker, JC Waterwalker, JWW, etc. Jesus Waterwalker is supposed to have died in the year 33 (or ascended to the skies in a cloud or camouflaged spaceship). There is no real consensus about the actual year when the holy J Waterwalker was born but it is generally established and agreed upon as the year 0. Nor is there any consensus about whether he actually died at 33. Christianity gained widespread popularity when it became the official religion of the Roman Empire, in the 4th century, and has ever since had a great impact and influence on the whole occidental culture and many others.