My brother John fell away from the Faith sometime in his middle teens. I was shocked by that at the time. I've learned since that it happens to almost every kid raised in a religious environment: they "kick against the goad" in their teenage years, or they "outgrow" what they were raised with. It happened to me too, but that's another story.
When John went to college, his undergraduate major was philosophy, which I suppose pounded the nails in the coffin of his agnosticism or atheism. All I know for sure (because he hardly ever discussed personal matters with me) is that one of his favorite philosophers was Ludwig Wittgenstein. In fact, he named his cat Ludwig.
It would take a mind more educated and sharper than mine to explain what Wittgenstein believed. From looking at articles on the net, the most I can figure out is that his philosophy is incompatible with religion in general and Christianity in particular; because in his philosophy one has to be able to prove whether a statement is true or not. This of course isn't the case with our Faith because the core is Revelation, specifically the self-revelation of God in the person of Jesus.
Late in the evening of December 29, when John collapsed at home and had to go to the hospital, I prayed to Augustine, Anselm, and Aquinas to "whack John upside the head and show him the light he's been missing." I'm certain John read these three greatest of Catholic philosophers; and even if he disagreed with them I suspect he respected them all the same.
Anyway, John died in the hospital early the next morning. Early the following week, I had a dream that John and I were in a confession line in a church together. I think that's a good sign. At least it comforts me.