When C. S. Lewis was a teenager, he became an atheist. As an atheist, he was a man of his time, because atheism was the current intellectual fashion. But not long after Lewis came to Oxford, he met a man who was to change the direction of his thought for the rest of his life: Owen Barfield.

Barfield wasn't a Christian, but he believed in God and the spiritual world, and so Lewis disagreed with him over just about everything. In fact, Lewis describes their conversations over the next few years as "the Great War."

Barfield moved the atheistic Lewis towards faith by helping him realize that all people tend to accept the prevailing beliefs of their age without much thought. Lewis says,

[Barfield] made short work of what I have called my "chronological snobbery," the uncritical acceptance of the intellectual climate common to our own age and the assumption that whatever has gone out of date is on that account discredited. You must find out why it went out of date. Was it ever refuted (and if so by whom, where, and how conclusively) or did it merely die away as fashions do? If the latter, this tells us nothing about its truth or falsehood. From seeing this, one passes to the realization that our own age is also "a period," and certainly has, like all periods, its own characteristic illusions. They are likeliest to lurk in those widespread assumptions which are so ingrained in the age that no one dares to attack or feels it necessary to defend them.

This realization helped Lewis realize that just because almost everyone in the culture around us believes a certain idea, that doesn't mean that it's true. Eventually, when Lewis looked at the evidence for the truth of Christianity, he found its foundation to be strong, and he was compelled to believe.