Growing up in an evangelical Protestant home, I recall my parents occasionally lamenting the defection of a recent convert. In some cases, they had spent months, even years, making new Christians — only to have some of them return their old ways of life.

This puzzled me. How could people leave the Truth? Why would anyone want to?

The seemingly clear vision of youth, inevitably tested, either dims or deepens. The way and the why of how faith comes, goes, withers or grows is a mystery. And the theological virtues of faith and hope can be fragile, just as the desires and secrets of the human heart can be elusive.

In Bible college, I became friends with Ben, a brilliant student with a passion for learning and for truth. We talked, prayed, laughed and cried together, sharing our passion for Jesus. We discussed our common love for art and literature, and Ben introduced me to the work of great authors, including Walker Percy, the Catholic novelist.

Today, Ben is not a Christian. After Bible college, he moved to London, earning a doctorate in philosophy. Out of touch for a while, we reconnected through the Internet and email. Both of us were walking away from evangelical Protestantism. I was on the path to Rome, and he was on the road to doubt. He began to openly scoff at the doctrine of the Trinity and the historical validity of much of the Bible.

Distance and sporadic correspondence made it difficult to gauge what was transpiring in Ben's life. Then, after another long silence, he emailed me, his bitterness obvious: He was getting divorced. Soon thereafter, he flatly stated: “I am no longer a Christian.” He added: “While no longer holding to the dogma of Christianity, this is not because I wish to deny its claims, but merely because I am not swayed by them.”

What had happened? How could Ben leave the truth? His faith, so vibrant years before, had died.

Two years ago, I met Bob and Susan, a young couple who had walked a long, tortured path in a short amount of time. They attended an evangelical Bible college together, married and then, inexplicably, in the midst of a personal crisis, embraced Wicca. After two years, they returned to Protestantism, but flirted with Judaism and Eastern religions.

Then, dramatically, they decided upon the Catholic faith. But on the cusp of entering the Church, they began doubting the validity of Vatican II and the Church's ecumenical endeavors. At the last moment, they changed their minds back again and became Catholic. But six months later, they began attending a Society of St. Piux X parish, explaining to bewildered friends that they had found the “true Church.” For whatever reason, Bob and Susan cannot fully embrace and hold onto the gift of faith.

As an apologist, I find it easy to think of an endless list of reasons to be Christian. As a convert, I know arguments why the Catholic Church is the true Church of Christ. But while faith and reason are not enemies, faith is ultimately a gift, a grace. And the cares of the world — fear, bitterness, pride — eat away at that precious gift.

Pride is often the strongest enemy of faith. Once humility and gratitude are trampled down, the temptation to rely on our natural abilities begins to swell. But the human mind and heart are dark without the supernatural light of faith. “We walk by faith, not by sight,” writes St. Paul. That doesn't mean life is easy, but it makes eternal life possible.

Carl E. Olson is co-author, with Sandra Miesel, of The Da Vinci Hoax.