How Keller defines Christianity

What is Christianity? For our purposes, I'll define Christianity as the body of believers who assent to these great ecumenical creeds. They believe that the triune God created the world, that humanity has fallen into sin and evil, that God has returned to rescue us in Jesus Christ, that in his death and resurrection Jesus accomplished our salvation for us so we can be received by grace, that he established the church ,his people, as the vehicle through which he continues his mission of rescue, reconciliation, and salvation, and that at the end of time Jesus will return to renew the heavens and the earth, removing all evil, injustice, sin, and death from the world.

All Christians believe all this - but no Christians believe just this. As soon as you ask, "How does the church act as vehicle for Jesus's work in the world?" and "How does Jesus's death accomplish our salvation?" and "How are we received by grace?" Catholic, Orthodox, and Protestant Christians will give you different answers. Despite the claims of many to be such, there are no truly "generic" nondenominational Christians. Everyone has to answer these "how" questions in order to live a Christian life, and those answers immediately put you into one tradition and denomination or another.

- Tim Keller, The Reason for God: Belief in an Age of Skepticism, p. 116/7