In the course of my studies I've come to realize that a lot of people have no idea what Christianity really means -- even people who profess to be Christian really don't understand what it means, and this is the real disappointment as the North American church has done a fantastic job of conforming the gospel to our culture when we should be aiming to conform our culture to the gospel (in other words, if we try to conform Truth, we end up with a twisted view of the truth or an outright lie). In a later posting I'll tackle the issue of truth (what it is and why it matters); suffice it to say for now I believe that truth is not subjective as relativism claims.
To begin to understand the origin of Christianity, you need to understand the orthodoxy of Christianity (the theories, doctrines, and practices that defined Christianity in the early church and are the pillars of Christian theology today). These statements or creeds are inviolate -- they are the foundations of Christianity and without them you may have something that resembles Christianity, but really isn't Christianity. There are two statements of faith that define Christianity: the Apostles' Creed and the Nicene Creed.
The Apostles' Creed
1. I believe in God the Father almighty, creator of heaven and earth. 2. I believe in Jesus Christ, his only Son, our Lord. 3. He was conceived by the power of the Holy Spirit and born of the Virgin Mary. 4. Under Pontius Pilate, He was crucified, died, and was buried. 5. He descended to the dead. On the third day he rose again. 6. He ascended into heaven and is seated at the right hand of the Father. 7. He will come again to judge the living and the dead. 8. I believe in the Holy Spirit, 9. the holy catholic Church, the communion of saints, 10. the forgiveness of sins, 11. the resurrection of the body, 12. and the life everlasting. Amen.
Note: I'm not catholic and this doesn't mean "catholic church" in the sense that we understand it today (not to be confused with the "Catholic Church" as an organized religion today). It comes from the Greek words kata (with respect to) and holos (whole) so the meaning of "catholic" in this statement is the universal (katholikos) church (or, to put it into our language today, "the holy world-wide united christian church" (again, not to be confused with the "United Church" as an organized religion today)).
The Nicene Creed
We 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 only begotten Son of God, begotten of his Father before all worlds, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us and for our salvation came down from heaven, and was incarnate by the Holy Spirit of the Virgin Mary, and was made man; and was crucified also for us under Pontius Pilate; he suffered and was buried; and the third day he rose again according to the Scriptures, and ascended into heaven, and is seated at the right hand of the Father; and he shall come again, with glory, to judge both the living and the dead; whose kingdom shall have no end. And I believe in the Holy Spirit, the Lord, and giver of life, who proceeds from the Father and the Son; who with the Father and the Son together is worshipped and glorified; who spoke by the prophets; and we believe in the one holy catholic and apostolic church; we acknowledge one baptism for the forgiveness of sins; and we look for the resurrection of the dead, and the life of the world to come.
Note that the Nicene Creed was initially written in 325 AD at the first council of Nicea, and expanded (and slightly re-arranged) at the first council of Constantinople in 381 AD. This creed was considered the Symbol of Faith.
Both of these creeds are foundational to Christianity. To take away or change any of these statements removes the foundational material of the faith and fundamentally changes the faith to something that is like Christianity, but is not, in fact, Christianity. For instance, Jehovah's Witnesses, despite having a similar Bible and similar beliefs, are not Christian -- they deny the triune nature of God and deny that Jesus is God, both of which are foundational truths to the Christian belief. Therefore a Jehovah's Witness cannot (and will not) claim to be Christian, because the foundations of the Jehovah's Witness faith are drastically and fundamentally different (not meaning to pick on Jehovah's Witnesses here, but they are probably the closest almost-but-definitely-not-Christian faith I can think of).
I want to reiterate what I noted above about the "catholic church" because I think it's very important to note, particularly for readers that are not Christian and don't understand the denominational differences that various churches have. I like the way that Charles Colson and Harold Fickett explain it in their book "The Faith"1:
The Church is one because all true Christians, while we participate in different confessing congregations, are part of one body. That body is holy because its essential nature is found in Christ. The Church is catholic because it is universal, which is what catholic means -- the Church is open to everyone. Finally, the Church is apostolic, which means that its teachings are those of the apostles. We have not invented a religion. We are part of the faith God revealed.
Finally, and perhaps most important to me, these are my statements of faith.