Francis August Schaeffer was an American evangelical theologian, philosopher, and Presbyterian pastor. He co-founded the L'Abri community in Switzerland with his wife, Edith Schaeffer, a prolific author in her own right. Schaeffer opposed theological modernism, which prefers rationalism and religious experience over following religious traditions based on revelation. He promoted what he claimed was a more historic Protestant faith and a presuppositional approach to Christian apologetics, which he believed would answer the questions of the age.

Schaeffer distinguished between the terms rationalism and rational: "Humanism in the larger, more inclusive sense is the system whereby men and women, beginning absolutely by themselves, try rationally to build out from themselves, having only Man as their integration point, to find all knowledge, meaning, and value. We must also ensure that the word rationalism, which means the same thing as humanism in the wider sense, is not confused with the word rational. Rational means that the things which are about us are not contrary to reason; or, to put it another way, man's aspiration of reason is valid. And so the Judeo-Christian position is rational, but it is the very antithesis of rationalism."