It is entirely wrong to place the rationalism of the Enlightenment in contradiction to pietistic mysticism. It is popular nonsense that reason and mysticism are the two great opposites. Historically, Pietism and the Enlightenment both fought against Orthodoxy. The subjectivity of Pietism, or the doctrine of the ‘inner light’ in Quakerism and the other ecstatic movements, has the character of immediacy or autonomy against the authority of the church. To put it more sharply, modern rational autonomy is a child of the mystical autonomy of the doctrine of the inner light.
Paul Tillich, A History of Christian Thought (New York: Simon and Schuster, 1967), 286.