[Ministers] must preach God's word in evidence and demonstration of the spirit of God. For he that is God's angel, the spirit of that God must speak in him. Now to speak in the demonstration of God's Spirit is to speak in such plainness, and yet such a powerfulness, as that the capacities of the simplest may perceive, not man, but God teaching them in that plainness; and the conscience of the mightiest may feel, not man, but God reproving them in that powerfulness. That this is so, appears by Saint Paul: If a man prophesies aright (says the Holy Spirit) the unlearned or unbelieving man comes in, he thinks his secret faults are disclosed and laid open, he thinks all men see his nakedness, and does reprove him for it; he therefore falls down and says, 'surely God speaks in this man'.
In which words, observe an admirable plainness, and an admirable powerfulness (which a man would think could not so well stand together). First, plainness: for whereas the unlearned man perceives his faults discovered, it follows necessarily he must needs understand; and if an unlearned man understand it, then consequently it must needs be plain. Second, powerfulness; in that his conscience is so convinced, his secret faults so disclosed, and his very heart ripped up, that he says, 'Certainly God speaks in this man'. This is the evidence and demonstration of God's Spirit.
William Perkins, The Workes: Duties & Dignities of the Ministry, Vol. III (John Legatt: London, 1618) 430.