Here’s the C.S. Lewis quote:
Barfield never made me an Anthroposophist, but his counterattacks destroyed forever two elements in my own thought. In the first place he 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the uncritical acceptance of the intellectual climate common to our own age and the assumption that whatever has gone out of date is on that account discredited. You must find why it went out of date.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.
And here’s the post where I found it, and that gives the quote some context: Piers Morgan: King of the Chronological Snobs | The American Spectator.