St. Ambrose shows by instances that the places in which those words were spoken help to the understanding of the words of the Lord; he shows that Christ uttered the passage quoted from St. John in Solomon's porch, by which is signified the mind of a wise man, for he says that Christ would not have uttered this saying in the heart of a foolish or contentious man. He goes on to say that Christ is stoned by those who believe not these words, and as the keys of heaven were given to Peter for his confession of them, so Iscariot, because he believed not the same, perished evilly. He takes this opportunity to inveigh against the Jews who bought the Son of God and sold Joseph. He explains the price paid for each mystically; and having in the same manner expounded the murmuring of the traitor concerning Magdalene's ointment, he adds that Christ is bought in one way by heretics in another way by Catholics, and that those in vain take to themselves the name of Christians who sever the Spirit from the Father.
118. It is worth while to notice in what place the Lord held this discussion, for His utterances are often [better] estimated by the kind of places in which He conversed. When about to fast, He is led (as we read) into the wilderness to render vain the devil's temptations. For although it deserves praise to have lived temperately in the midst of abundance, yet the enticements of temptation are more frequent among riches and pleasures. Then the tempter, in order to try Him, promises Him abundance, and the Lord in order to overcome cherishes hunger. Now I do not deny that temperance can exist in the midst of riches; but although he who navigates the sea often escapes, yet he is more exposed to peril than he who will not go to sea.
119. Let us consider some other points. When about to promise the kingdom of heaven, Jesus went up into a mountain. At another time He leads His disciples through the grain-fields, when about to sow in their minds the crop of heavenly precepts, so that a plentiful harvest of souls should ripen. When about to consummate the work of the flesh which He had taken, having now seen perfection in His disciples, whom He had established upon the root of His words, He enters a garden, that He might plant the young olive-trees in the house of the Lord, and that He might water the just flourishing like a palm-tree, and the fruitful vine with the stream of His Blood.
120. In this passage too He was walking, as we read, in Solomon's porch on the day of the dedication, that is, Christ was walking in the breast of the wise and prudent, to dedicate his good affection to Himself. What that porch was the prophet teaches, saying: “I will walk in the midst of Your house in the innocency of my heart.” So, then, we have in our own selves the house of God, we have the halls, we have also the porches, and we have the courts, for it is written: “Let your waters flow abroad in your courts.” Open, then, this porch of your heart to the Word of God, Who says to you: “Open your mouth wide and I will fill it.”
121. Let us, therefore, hear what the Word of God, walking in the heart of the wise and peaceful, says: “I and My Father are One.” He will not say this in the breast of the unquiet and foolish, for “the natural man receives not the things of the Spirit of God, for they are foolishness unto him.” The narrow breasts of sinners do not take in the greatness of the faith. Lastly, the Jews hearing, “I and the Father are One, took up stones to stone Him.”
122. He who cannot listen to this is a Jew; he who cannot listen to this stones Christ with the stones of his treachery, rougher than any rock, and if you believe me, he wounds Christ. For although He cannot now feel a wound: “For now henceforth we know not Christ after the flesh,” yet He Who rejoices in the love of the Church is stoned by the impiety of the Arians.
123. “The law of Your mouth, O Lord, is good unto me, I keep Your commandments.” You have Yourself said that You are one with the Father. Because Peter believed this, he received the keys of the kingdom of heaven, and without anxiety for himself forgave sins. Judas, because he believed not this, strangled himself with the cord of his own wickedness. O the hard stones of unbelieving words! O the unseemly cord of the betrayer, and the still more hideous purchase-money of the Jews! O hateful money wherewith either the just is bought for death, or sold! Joseph was sold, Jesus Christ was bought, the one to slavery, the Other to death. O detestable inheritance, O deadly sale, which either sells a brother to suffering or sets a price on the Lord to destroy Him, the Purchaser of the salvation of all.
124. The Jews did violence to two things which are chief of all, faith and duty, and in each to Christ the Author of faith and duty. For both in the patriarch Joseph was there a type of Christ, and Christ Himself came in the truth of His Body, “Who counted it not robbery that He should be equal with God, but took on Him the form of a servant,” because of our fall, that is to say, taking slavery upon Himself and not shrinking from suffering.
125. In one place the sale is for twenty pieces, in the other for thirty. For how could His true price be apprehended, Whose value cannot be limited? There is error in the price because there is error in the inquiry. The sale is for twenty pieces in the Old Testament, for thirty in the New; for the Truth is of more value than the type, Grace is more generous than training, the Presence is better than the Law, for the Law promised the Coming, the Coming fulfilled the Law.
126. The Ishmaelites made their purchase for twenty pieces, the Jews for thirty. And this is no trivial figure. The faithless are more lavish for iniquity than the faithful for salvation. It is, however, fitting to consider the quality of each agreement. Twenty pieces are the price of him sold to slavery, thirty pieces of Him delivered to the Cross. For although the Mysteries of the Incarnation and of the Passion must be in like manner matters of amazement, yet the fulfilment of faith is in the Mystery of the Passion. I do not indeed value less the birth from the holy Virgin, but I receive even more gratefully the Mystery of the sacred Body. What is more full of mercy than that He should forgive me the wrongs done to Himself? But it is even fuller measure that He gave us so great a gift, that He Who was not to die because He was God, should die by our death, that we might live by His Spirit.
127. Lastly, it was not without meaning that Judas Iscariot valued that ointment at three hundred pence, which seems certainly by the statement of the price itself to set forth the Lord's cross. Whence, too, the Lord says: “For she, pouring this ointment on My body, did it for My burial.” Why, then, did Judas value this at so high a rate? Because remission of sins is of more value to sinners, and forgiveness seems to be more precious. Lastly, you find it written: “To whom much is forgiven the same loves more.” Therefore sinners themselves also confess the grace of the Lord's Passion which they have lost, and they bear witness to Christ who persecuted Him.
128. Or because, “into a malicious soul wisdom does not enter,” the evil disposition of the traitor uttered this, and he valued the suffering of the Lord's body at a dearer rate, that by the immensity of the price he might draw all away from the faith. And therefore the Lord offered Himself without price, that the necessity of poverty might hold no one back from Christ. The patriarchs sold Him for a small price that all might buy. Isaiah said: “You that have no money go buy and drink; eat ye without money,” that he might gain him who had no money. O traitor Judas, you value the ointment of His Passion at three hundred pence, and sellest His Passion for thirty pence. Profuse in valuing, mean in selling.
Source: On the Holy Spirit (New Advent)