Garry Wills examines Santorum's claim:

I think it inevitable that questioning of childhood beliefs should take place at various stages of adolescence. This does not happen in junior year or senior year on campus. It is part of a long process called growing up. … An unquestioned faith is not faith but rote recitation. The opposite of such questioning is not deep belief but arrested development.

Eric Reitan has more:

Scholars in academia engage in critical and reason-based thinking on every conceivable topic of human interest—including religion and religious dogma. As such, conservative Christian teachings are not immune to the academy’s critical attention. But for those who view these teachings as identical with the revealed truth of God, such critical attention will be seen as nothing but hubris; nothing but the vaunting of human pride over God’s word.  

Learning is, in this view, a spiritual vulnerability. Those who are less educated are less likely to critically assess the pronouncements of a religious authority—a critical assessment which, given Santorum’s vision of cosmic holy war, is a Satanic attack on divine truth.