Francis August Schaeffer (1912-1984) was an American Evangelical Christian theologian, philosopher, and Presbyterian pastor. He is most famous for his writings and his establishment of the L'Abri community in Switzerland. Opposed to theological modernism, Schaeffer promoted a more historic Protestant faith and a presuppositional approach to Christian apologetics, which he believed would answer the questions of the age. (from InfoGalactic)
For those unfamiliar with Schaeffer, we recommend either reading the core of his work in order: The God Who Is There, Escape From Reason, and He Is There & He Is Not Silent - or the classic How Should We Then Live? - likely his most popular work.