The first half of Tim Keller's The Reason for God deals with some common objections secular Westerners have to Christianity.
How could a good God allow suffering?
You can't take the Bible literally.
The church has been responsible for so much injustice.
Hasn't science disproved Christianity?
Questions and statements like these constitute what Keller has called elsewhere "defeater beliefs" -- beliefs that have to be confronted before you can gain a hearing for the gospel message. The second half of the book is devoted to making the positive case for the Christian faith.
But before getting to that, Keller anticipates a question in an Intermission chapter -- How do you define Christianity? After all Christians are a veritable hodge-podge of beliefs and practices. Imagine the disorientation of someone attending a Catholic Mass one day and a Pentecostal service the next. He could be forgiven for thinking he had experienced two different religions. Nevertheless, Keller writes that it's possible to identify a core of beliefs and practices that make up what C.S. Lewis called "mere Christianity."
. . . 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. . .
That's good. Also included in the Christian view of reality expressed in the creeds are humanity's fall into sin, salvation through the death and resurrection of Jesus, and the establishment of the church to continue Christ's mission until he returns to judge sin, remove evil, and usher in the new heavens and earth.
Of course, once you start asking the "how" questions (How does Jesus's death accomplish our salvation?, etc.) you will get significantly different answers from different traditions and denominations. Also, since Christianity is spread across every region of the world it will look different depending on the context. So even though there are no truly "generic" Christians there is a definable body of beliefs that define Christianity. Even believers who've never heard of the creeds listed above are defined by the beliefs which they articulate, and the episodes of church history from which they emerged. The Roman Catholic and the "no creed but the Bible" fundamentalist have more in common than they might think!
Quote from this edition of The Reason for God: Belief in an Age of Skepticism, pp. 120-1