On September 8, 1947, C. S. Lewis's stature as an apologist was established when he appeared on the cover of Time magazine. The accompanying article declared him to be "one of the most influential spokesmen for Christianity in the English-speaking world." The intervening years have only added to the belief that Lewis was the greatest apologist of the 20th century, and millions of lives have been changed by reading his Mere Christianity.

In 1797, 150 years before Lewis appeared on the cover of Time, the English anti-slavery reformer William Wilberforce published the Mere Christianity of his time: A Practical View of Christianity. As with Lewis' book, A Practical View eventually sold millions of copies. Like Lewis, Wilberforce had a gift for setting forth what biographer John Pollock calls "the intellectual heart of Christianity."

Wilberforce asked unbelievers "one plain question" that was hard to dismiss. It led many readers to reconsider their preconceptions about Christianity.

If Christianity be not in [your] estimation true, yet is there not at least a presumption in its favour sufficient to entitle it to a serious examination; from its having been embraced, (and that not blindly and implicitly, but upon full inquiry and deep consideration,) by Bacon and Milton, and Locke and Newton, and much the greater part of those, who, by the reach of their understandings, or the extent of their knowledge, and by the freedom of their minds, and their daring to combat existing prejudices, have called forth the respect and admiration of mankind?

Such questions had a profound effect on the Scottish philosopher Thomas Chalmers. "About the year 1811," he wrote, "I had Wilberforce's View put into my hands, and, as I got on in reading it, I ...