1 Corinthians 14:25

And thus are the secrets of his heart made manifest. Out of the gift of discerning of spirits, or because God directs the tongue of the prophet, i.e., the preacher, the most hidden sins of his heart will be described and reproved, and the man will think that the preacher speaks as a prophet to him in particular. It is evident from this that this was a common occurrence; it is also evident that these teachers and preachers were, strictly speaking, real prophets.

There is a parallel case in the, life of S. Augustine by Possidonius (c. 15), where it is said that on one occasion S. Augustine left the subject that he had decided to speak on, and discoursed on Manichæism. This led to the conversion of a certain Manichæan, who chanced to be present, as S. Augustine afterwards learnt. He believed it to be due to the direct guidance of God. Hence (de Doct. Christ. lib. iv. c. 15) he says that prayer should always be offered to God before preaching, that He would direct the mind and tongue of the preacher suitably to the capacity and disposition of the audience.

Others, however, understand "the secrets of his heart" to mean the sins which the unbeliever or unlearned has, but which he does not know to be sins, e.g., when he does not know that idolatry and fornication are sinful. He will learn this when he hears the prophet discoursing about them, and condemning them as sinful. But the first meaning is the best.