John Chrysostom — as excerpted in the Catena Aurea on Matthew 13:45-46

“The Gospel preaching not only offers manifold gain as a treasure, but is precious as a pearl; wherefore after the parable concerning the treasure, He gives that concerning the pearl. And in preaching, two things are required, namely, to be detached from the business of this life, and to be watchful, which are denoted by this merchantman. Truth moreover is one, and not manifold, and for this reason it is one pearl that is said to be found. And as one who is possessed of a pearl, himself indeed knows of his wealth, but is not known to others, ofttimes concealing it in his hand because of its small bulk, so it is in the preaching of the Gospel; they who possess it know that they are rich, the unbelievers, not knowing of this treasure, know not of our wealth. Jerome; By the goodly pearls may be understood the Law and the Prophets. Hear then Marcion and Manichæus; the good pearls are the Law and the Prophets. One pearl, the most precious of all, is the knowledge of the Saviour and the sacrament of His passion and resurrection, which when the merchantman has found, like Paul the Apostle, he straightway despises all the mysteries of the Law and the Prophets and the old observances in which he had lived blameless, counting them as dung that he may win Christ. (Phil. 3:8.) Not that the finding of a new pearl is the condemnation of the old pearls, but that in omparison of that, all other pearls are worthless.”