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.
This should be read in the light of other factors too. Historically Pietism was a movement within Lutheranism begun by Philip Spener. As such it did *not* fight against orthodoxy in the sense of Orthodox doctrine; Spener professed all the teachings of Lutheran orthodoxy. Spener and Pietism did however oppose the polemical, hair-splitting, doctrinaire nature of 17th century Lutheran Christianity (there was a similar phenomenon in reformed Christianity), redirecting energies towards edification and good works.
Pietism did have its weaknesses and might be seen as part of a broader mood of subjectivism, mysticism and an emphasis on the subjective side (experience etc.) rather than the objective authority of doctrine and God’s Word. However the Orthodoxy to which it responded also had its weaknesses and there is a good argument for saying that it was that extreme orthodoxy which inevitably produced the exaggerated response.
Also, it seems somewhat out of place that Paul Tillich is being marshalled in support of Reformation Christianity.