It was one of the great mistakes of the Pietism of the seventeenth and eighteenth centuries that, in seeking the assurance of faith, or of salvation, it divorced itself too much from the Word of God. The basis of assurance was sought, not in the objective promises of the gospel, but in the subjective experiences of believers. The knowledge of the experiences that were made the touch-stone of faith, was not not gathered from the Word of God, but was obtained by an inductive study of the subjective states and affections of believers.
Louis Berkhof. The Assurance of Faith. (hier gelesen)