I think it is an obvious truth that people don't give up their faith for intellectual reasons. If this were true, there would be no intellectuals of faith. People who claim that they abandoned the faith because it was not intellectually compelling to them are perhaps telling only part of the story. Yet, as brilliant as some apostates are, I'm not sure anyone would put their native intelligence up against that of someone like the late Cambridge Lady Margaret's Professor of Divinity Graham Stanton whose faith was well attested to the day he died. Likewise, Gordon Fee knows so much about textual criticism that he could cut the legs off any argument against the reliability of the New Testament text, off the top of his head. Some of history's greatest minds were people of Christian faith, people who are intellectual giants in comparison with even the best of moderns.
No, people don't quit believing for intellectual reasons. They may make such claims, but to do so they must presume to understand facts and history and logic better than people such as Carl F. H. Henry, Francis Schaeffer, F.F. Bruce, or Alvin Plantinga, and such claims will not stand. They are but mere excuses.
People don't forfeit their faith for intellectual reasons. Rather, people withdraw their faith in Christ because in life's circumstances, they become disappointed with Jesus and God personally. People who abandon Christ in the storms of life often resort to the intellectual excuse because it is easier to attack one's former faith than to delve deeply into a serious analysis of their personal relationship with Jesus.
Apostasy is not a new phenomenon. John the Revelator saw a great tide of persecution about to engulf the seven churches in Asia (Rev 2-3), and he perceived that their spiritual weaknesses made them vulnerable to apostasy. The only way that these churches would withstand such a great persecution was to renew their relationship with Christ (cf. John's condemnation of the church of Ephesus for having left its first love). It is to those who have fallen so far that Jesus says the words, "Behold I stand at the door and knock."