Christianity is the world's most popular religion with over 2 billion adherents worldwide, with most of Europe, Southern Africa and the Americas being predominantly Christian. It is increasing in parts of the developing world, while in areas such as the north of North America and Australia the numbers are in decline.

Christians believe that Jesus of Nazareth was the Messiah promised in the Old Testament. Little is known about his early life, but it is said after his baptism by John the Baptist he had a vision in which he received God's blessing. He is said to have been born to Mary through Immaculate Conception, which despite common misconceptions does not exactly always mean Mary was a virgin; rather that he was born without sin. He began healing, miracle-working and spreading the will of God and condemning religious hypocrisy. As well as working with the masses he chose 12 disciples to teach privately, who would carry on his work. During this time, however, the opposition against Jesus began to rise and he was ultimately betrayed by Judas (a disciple) and executed by the Romans in a manner reserved for the most contemptible of criminals. He was nailed to a cross. His followers scattered, dismayed at what to do next, but when some women visited his grave he was found to be gone and an angel told them he had risen from the death. The disciples were initially skeptical, but came to believe after seeing him appearing to them and ascending to heaven.

Christians believe that Jesus was the Son of God, and that he died to restore humanity's broken relationship with God. In Christianity there is one God, who is the creator of the universe and all knowing, and has three elements. The father, the son and the Holy Spirit. This is called the doctrine of the Trinity. God loves unconditionally and humans can get to know him by worshipping and loving him. Original sin is part of Augustine doctrine and says that everyone is born sinful and goes back to when Adam and Eve sinned against God in the Garden of Eden, when they ate the forbidden fruit. Original sin explains why there is so much suffering and imperfection in a world created by a perfect God. One of the oldest catechisms states that the purpose of a Christian life is to worship and glorify God, as it is what we were created for. Altruism is central to Christian values and is the focus in parts of the Gospel such as Sermon on the Mount and Sermon on the Plain. "Love thy neighbor as you love yourself" is a great example of this, as a phrase in the main Christian holy book- the Bible.

While all Christians believe that Jesus was the Son of God, and that following in his footsteps by being forgiving, loving and giving to the poor is the correct way to conduct one’s life, there are a wide range of denominations. The most popular branches worldwide are the Catholic Church and the Eastern Orthodox Church, who split off from each other in the schism in the 11th century, and various forms of Protestantism. This has created a range of views on the afterlife, what will happen and how to enter heaven or hell and these are strongly influenced by the writing in the book of Revelations/. Some Calvinist thinkers believe that our fate is predetermined wholly and the belief that we can simply earn our way into heaven is something of a folk tale. There are differing conceptions of hell, and seventh day Adventists and Jehovah's Witnesses believe that while the good are sent to heaven, sinners are destroyed immediately, rather than tortured in hell until the end of time. Catholics believe that the dead are judged upon death, and the free of sin and righteous go straight to heaven, while sinners go to hell. They see this as the sinner's self-exclusion from God, rather than the punishment given from God. Those who die in a state of grace but still carry venial sin are sent to Purgatory, where they are purified in order to enter heaven. Each denomination has different terms for the 'end times' i.e. Armageddon, Judgment day, with each having an elaborate idea of what will happen.

Christianity has a multitude of holy days, but a few are well known and celebrated widely, even by secular people as part of cultural tradition. Christmas is perhaps the most well-known and celebrates the birth of Jesus Christ (called the nativity and described in the Gospel) on the 25th of December, although Eastern Orthodox communities will celebrate on the 7th of January instead. The Gospels do not mention the date, but in the 4th century AD Pope Julius set the date. Christmas isn't exclusively Christian and incorporates many other religious traditions that date back to ancient Greeks and the Druids.

Lent occurs in the first 40 days before Easter, beginning on Ash Wednesday, and is a period of thought and self-reflection before the festivities of Easter. During Lent, one might give up certain foods or vices, such as meat and fish. While Lent recalls the events leading up to the crucifixion of Jesus, Easter celebrates his resurrection. The date of Easter changes yearly for many denominations, and churches are filled with hymns and flowers. The symbol of rebirth is carried further, and it is common for shops to stock chocolate Easter eggs as part of this.

Because of the breadth of belief, the head of the spiritual community or senior religious officials is varied. Protestant churches will often have a vicar or similar, and Catholics will have a priest or a higher ranking Bishop. The Archbishop is a strong figurehead in England and for the Catholic world there is, of course the Pope. So far there have 266 Popes, and the latest, Pope Francis is the latest and incredibly controversial. One of the issues many Christians face is marrying their beliefs with modern issues such as contraception, abortion and same sex marriage, all of which this Pope has expressed liberal views on.