This is a bit like the toothpaste in the tube problem. There was a time when exclusive humanism was implausible to the vast majority of people. They just did not live in a world where denial of the supernatural made plausible sense of the world to them. However, once exclusive humanism did become plausible, that humanism became an ever-present, haunting possibility. Christian belief has therefore become more challenging because other approaches to reality now exist as plausible alternatives which didn't exist before.
What Smith does early on in the book is point out that the problem of belief cuts both ways. Yes, it is harder to be a Christian today than it has ever been before, but it's also equally hard to be an unbeliever, too. He uses the work of David Foster Wallace as representative of the fact that life is much more muddy, grey, and difficult than either the New Atheists or Religious Fundamentalists would portray it to be. There is an ongoing transcendent haunting that takes place in the heart of the skeptic. People yearn for transcendence, but they largely despise the source of transcendence. This yearning for transcendence is everywhere around us and easily demonstrable. There is truly a deep, yearning hunger within humanity for more than what we can see. But there is a cynicism and an infectious pessimism that essentially says that we cannot have this transcendence that we want because we no longer find it to be plausible. This yearning, then, is interpreted by some (one thinks of Richard Dawkins) as a sickness or a parasite that humanity must find a way to shake.
Viewers of the TV show House may recall that the nihilistic lead character, Gregory House, goes into every situation with a head-first rationalism and almost always enters conversations with religious individuals trouncing them with rationalism and mocking them for their beliefs. But nearly every time he in some measure reaches the end of the episode and finds himself rebuffed for his cynicism and a bit in doubt about whether he really understands what life is like. This metaphysical tension never quite goes away on the show.
One episode in particular features a priest who doesn't believe in God. He is an atheist. In one poignant moment at the end of the episode, House, being his usual self, tells the priest, in spite of the massive amount of coincidences that went into his healing, "Don't worry your life will go back to sucking soon enough. Everything that happened can be rationally explained." To which the priest responds with a measure of doubt in his voice, "I know. It's just... a lot of coincidences." The atheist is shaken in his faith. The atheist, the skeptic, the unbeliever, will always be haunted by transcendence because he was made to know God and yet lives in a time in which it is culturally plausible to reject that being who can rationally account for transcendence.
This is what it is to inhabit the secular world today. It is hard to be a believer, sure, but it sure is hard to be an unbeliever, as well, constantly dogged by the memory of a transcendent and holy God. The people we meet need to hear that there are answers to their questions and they need to hear us without shame or embarrassment tell them that God's Word has the answers they are looking for. But they also need to hear us say that we know that the search for answers isn't always easy, and we don't always have the answers to every question, and that's okay. Part of the challenge of living in this day and age is that the answers don't always come easily.
