Finally Perkins speaks of the vitality of the Word of God (p. 647, The Art of Prophesying). It is endowed with virtue in its operation. This means that the Word has a vitality, a power to convince the hearers of its truth and to bring about that which it promises. We notice that others in the history of preaching have spoken of this virtue or vitality of God’s Word. It is a fundamental concept for understanding the Puritan School of preaching, because it is on this insight that the confidence of the preacher is built. He does not have to rely on the arts of oratory, although he may use them, because the power of the preaching is not in the preacher but in the Word itself.
Hughes Oliphant Old, The Reading and Preaching of the Scriptures in the Worship of the Christian Church: Vol IV, the Age of the Reformation, 265.