Reading the interview referred to in the last post sent me back to The Reason for God. Keller spends the first half of his book raising doubt about the usual objections to Christianity - asking the skeptics to be skeptical about what they take for granted. The second half is called "The Reasons for Faith." In between, in what he calls an intermission, he defines what he means by "Christianity." Here is part of what he says:
From the outside the various Christian churches and traditions can look extremely different, almost like distinct religions. This is partially because the public worship services look so different. It is also because…Christianity is the faith that is most spread across the cultures and regions of the world. Therefore it has assumed an enormous number of different cultural forms. Another reason that Christians look so different from one another is the great theological rifts that have occurred over the centuries. The first great division was between the eastern Greek and western Roman church in the eleventh century. Today these are known as the Eastern Orthodox and Roman Catholic churches. The second great schism was within the Western church, between Roman Catholicism and Protestantism.This is very much like C.S. Lewis's "mere Christianity" or what most of us would call "orthodoxy." The doctrinal differences that divide us into separate denominations are important [or, at least, many of them are], but their importance is in addition to - not instead of - what make us all Christians, and separates us from those who hold other creeds.
All Christians who take truth and doctrine seriously will agree that these differences between churches are highly significant. They make a major difference in how one's faith is held and practiced. Nevertheless, all Orthodox, Catholic, and Protestant Christians assent together to the great creeds of the first thousand years of church history, such as the Apostle's, Nicene, Chalcedonian, and Athanasian creeds. In these creeds the fundamental Christian view of reality is laid out. There is the classical expression of the Christian understanding of God as three-in-one. Belief in the Trinity creates a profoundly different view of the world from that of polytheists, non Trinitarian monotheists, and atheists, as I will show in Chapter 13. There is also a strong statement of the full deity and humanity of Jesus Christ in these creeds. Christians, therefore, do not look upon Jesus as one more teacher or prophet, but as Savior of the world. These teachings make Christians far more like than unlike one another.
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. ….