Christianity is a religion taking its name from Jesus Christ. Adherents are called Christians. It seems that at first the religion was called "The Way" and the adherents were called Nazarenes (after the city of Nazareth where Jesus lived). The name Christian arose in Antioch in the first century A. D. and its use spread, probably closer to mid-century since it is recorded in the Book of Acts (Acts:11:26)
Main Christian groups
The three largest self-governing bodies of Christians are:
- the Roman Catholic Church (approx. 1.1 billion baptized members, tracing their roots back to Saint Peter who they believed established the Church and the succession of the Popes as the spiritual authority of the Christian body of believers.)
- the Orthodox Churches (approx. 300 million baptized members) which hold ancient theological roots stretching back to the beginnings of Christianity. Different theological perspectives led to the Great Schism between the Western Roman Catholic Church and the Eastern Orthodox Catholic Church in A.D. 1054. In 1992 a statement of theological agreement was issued between the Eastern and Oriental Orthodox Churches; and although not universally recognized, most observers would now group these Churches together theologically.
- Protestantism, the largest Communion being the Anglicans (approx. 77 million baptized members). Protestantism has its origins in the European Reformation. It first broke away from the Roman Catholic Church under Martin Luther when differences over the nature of faith and works in the role of salvation could not be adequately reconciled as well as other practices that Luther saw in the Catholic Church at that time that he did not agree with. Other preachers and movements then followed Luther's example and also left the Catholic fold.
Theologians, over two millennia, have debated on a definitive summary of the Christian faith. While its interpretations vary drastically, probably the most commonly accepted statement of faith is the Nicea-Constantinopolitan Creed below:
I believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of Light; true God of true God; begotten, not made; of one essence with the Father, by whom all things were made; who for us men and for our salvation came down from heaven, and was incarnate of the Holy Spirit and the Virgin Mary, and became man. And He was crucified for us under Pontius Pilate, and suffered, and was buried. And the third day He rose again, according to the Scriptures; and ascended into heaven, and sits at the right hand of the Father; and He shall come again with glory to judge the living and the dead; whose Kingdom shall have no end. And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshiped and glorified; who spoke by the prophets. In one Holy, Catholic, and Apostolic Church. I acknowledge one baptism for the remission of sins. I look for the Resurrection of the dead, and the life of the world to come. Amen.
Christians and Christian denominations agree on many points of doctrine and disagree on others. According to an online Harris poll from 2003 99% of all American Christians believe in God, 96% in the resurrection of Jesus Christ, 93% in Heaven, 93% in the virgin birth, 92% in the survival of the soul after death, 82% in Hell, 50% in ghosts, 27% in astrology and 21% in reincarnation.  Note that the latter two beliefs are in opposition to the religious dogma of most Christian denominations.
Nonetheless, the Nicea-Constantinopolitan Creed offers a general overall picture of what Christian theology looks like, and serves as a useful outline.
The God of the Christians is a triune being. Though there is only one Divine nature there are three Divine Persons: The Father, the Son, and the Holy Spirit. The three Persons are collectively called the Trinity or the Holy Trinity. Christians reject the idea that they are polytheists because of the oneness of the divine nature (or essence). Though to non-Christians and even many Christians, it might seem like an inconsequential dogma, the doctrine of the Trinity is central to all of Christian theology and life. This is especially true in the relationship between God and human beings. The major theme of the Bible is love. In the Hebrew Old Testament the idea is expressed in the Hebrew word hessed, which is variously translated as loyal love, tender mercy, steadfast love, mercy, goodness, etc. in the New Testament the same idea is expressed in the Greek word agape, which is variously translated as love, compassion, charity, etc. The picture that the Biblical writers draw with these words is of a lover (God) who is entirely self-sufficient, needing nothing, and a beloved (human beings) in desperate need of salvation but unable to obtain it for himself. But God the lover is willing to suffer, knows He is going to suffer, even endure death to save the beloved human beings. This love of God for human beings is an extension of the love the Three Persons in the Trinity have for each other. Each loves the others infinitely. Their love for each other and for their creation is such that the Apostle John equates God and love, in an almost mathematical way saying, "God is love."
Additionally, the God of the Christians is the creator of all things, is everywhere present, exists in all times, is transcendent, all-knowing (omniscient), just, all-powerful (omnipotent).
Christians hold that during the reign of Caeser Augustus the Son (e.g. the second Person of the Trinity) took flesh from a virgin woman and was incarnate as a man. He was born in the town of Bethlehem and was given the name Jesus. At the age of thirty he was baptized by his cousin, the Prophet John, and began to preach in the area Palestine. About three years after his baptism, he raised his friend Lazarus from the dead, prompting the Jewish power establishment to plot Jesus death. Jesus was crucified. He came back from the dead and was seen by over 500 people. He ascended to heaven. The four Gospels contain the records of some of what Jesus did and said, but he did much more than those four books relate, as the Apostle John admitted in his Gospel.
The fundamental principle in Christian moral teaching is love and forgiveness, as expressed by the life and teachings of Jesus Christ and the New Testament. Jesus summarized his teachings in two commandments from the Old Testament:
"'You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.' This is the great and foremost commandment. The second is like it, 'You shall love your neighbor as yourself.' On these two commandments depend the whole Law and the Prophets." (Matthew 22:37-39; Deuteronomy 6:5; Leviticus 19:18)
Constant debate has resulted as to how a person should express love for God in their moral behavior. This moral dialogue found expression in the New Testament, where the Apostle Paul addressed such controversies as circumcision (Romans 2:25-29), eating meat that was sacrificed to pagan deities (1 Corinthians 8), speculating about myths and genealogies (1 Timothy 1:3-5), and observing ceremonial dates and seasons (Galatians 4:9-11).Regardless of a person's ethical interpretations, adherents commonly point to New Testament passages John 3:16 and 1 Corinthians 13:4-7 as scriptural depictions of love. The former states that
"God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life."The latter characterizes love, saying
"Love is patient, love is kind and is not jealous; love does not brag and is not arrogant, does not act unbecomingly; it does not seek its own, is not provoked, does not take into account a wrong suffered, does not rejoice in unrighteousness, but rejoices with the truth; bears all things, believes all things, hopes all things, endures all things."
Jesus Christ affirmed, "By this all men will know that you are My disciples, if you have love for one another." (John 13:35)
The Bible teaches that "all have sinned and fall short of the glory of God" (Romans 3:23). This is often interpreted to mean that everyone has displeased God and is now separated from him in a kind of alienation and enmity that results from the fundamental conflict between selfish human interests and God's interests (Romans 8:5-8; James 4:4).
However, Jesus offered a solution to this Biblical dilemma in that by repentance of sins and faith in him (Jesus), their sins would be forgiven. He said that "...the Son of man hath power on earth to forgive sins." (Mark 2:10) Jesus also said, "I came not to call the righteous, but sinners to repentance." (Mark 2:17, and "Verily I say unto you, All sins shall be forgiven unto the sons of men" (Mark 2:28)
Jesus Christ taught that "unless one is born again he cannot see the kingdom of God" (John 3:3). Reformed Christians often use the terms "saved" and "born again" interchangeably. Other Christians, notably the Roman Catholic Church and the Orthodox Church use the phrase born again as a synonym for baptized. "Jesus answered, Amen, amen, I say unto thee, Except a man be born of water and [of] the Spirit, he cannot enter into the kingdom of God." (John 3:5)
Christians are expected to continue living by Christ's teachings (John 8:31), as is appropriate for "children of Light" (Ephesians 5:8-10). Some believe that this is necessary in order to stay saved. However, this is a common misconception of the text. It is rather referring to proving that one is a child of God by their "fruit" (things that they do and how they behave). Christians in the Reformed tradition (following the teaching of the 16th century French lawyer John Calvin, as well as the faith outlined in the Belgic and Heidleberg Confessions) say that salvation is irrevocable and that it cannot be lost, if it were genuinely part of one's life to begin with. Reformed Christians (often called Calvinists) often point to Romans 8:38-39 as validation of their belief: "For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord" (Romans 8:38-39; NASB). According to Calvinists, the reason it cannot be lost by natural things is because salvation was obtained through a supernatural being, namely Jesus Christ. This does not negate Jesus' human side, only that he was both one-hundred percent God and one-hundred percent man, according to the Council of Chalcedon (A. D. 451). The Bible also makes it clear that mankind cannot earn their salvation, and that it is a free gift.
Resurrection of Jesus Christ
The resurrection of Jesus Christ is critical to the Christian faith. The Apostle Paul wrote, "if Christ has not been raised, then our preaching is vain, your faith also is vain" (I Cor:15:14). Traditionally, Christianity has believed in a physical resurrection of Jesus Christ. 
In recent history Christian apologists Gary Habermas is considered the foremost apologist for defending the resurrection of Jesus.  Other notable defenders of the resurrection include: William Lane Craig, Lee Strobel, Josh McDowell, Edwin M. Yamauchi, N.T. Wright and Michael Horner. 
In what is called the Great Commission, Jesus sent his disciples out into the world to preach the Gospel (literally "good news") and make disciples.
- Great Commission
- But the eleven disciples proceeded to Galilee, to the mountain which Jesus had designated. When they saw Him, they worshiped Him; but some were doubtful. And Jesus came up and spoke to them, saying, "All authority has been given to Me in heaven and on earth." Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age." --Matthew 28:16-20 NASB
It should be noted, however, that Jesus said it was the Holy Spirit, not man, who was sent to convict the world concerning sin and righteousness (John 16:8). This relieves Christians of needing to worry when they evangelize. God is the one who gets the credit with the growth and maturity of the Church: "So then neither the one who plants nor the one who waters is anything, but God who causes the growth" (1 Corinthians 3:7).
Denominations or branches of Christianity
- Roman Catholic Church
- Orthodox Church
- Christian apologetics (Defense of Christianity)
- Christianity and Science
- Conservative Christianity
- Persecution of Christians
- Red-letter Christian