Mateus 16
Bernard of Clairvaux
Mateus 16:19
Peter therefore shall enter, who received the keys. Yet not alone: for he will also introduce me, if he shall have willed it, and will exclude another whom perhaps he shall have willed, in the knowledge and power given to him from above. And what are these keys? The power of opening and closing, and the discernment between those to be excluded and those to be admitted. And the treasures are not in the serpent, but in Christ. And therefore the serpent could not give the knowledge which he did not have; but he who had it, gave it. Nor indeed could the serpent have the power which he did not receive; but he who received it, had it. Christ gave, Peter received, and was neither puffed up by the knowledge, nor to be cast down on account of the power. Why? Because in neither does he exalt himself against the knowledge of God, who desired none of these things apart from the knowledge of God. — Sermons on the Song of Songs, Sermon 69
Mateus 16:23
When he was speaking about his approaching death, Peter who loved him so dearly, tried to stand in the way. When, as you remember, he rebuked him, what was it but his imprudence that he was correcting? Finally what did he mean in saying: "You do not mind the things of God," except: you do not love wisely, you are following your human feeling in opposition to the divine plan. He even called him Satan because although it was in ignorance, he was impeding salvation in trying to prevent the Savior's death. — Sermons on the Song of Songs, Sermon 20
Mateus 16:25
And indeed more prudently do you lose it in order to keep it, than keep it in order to lose it. "For whoever would save his soul, shall lose it" (Mt 16:25). What do you say here, you observers of foods, neglecters of character? Hippocrates and his followers teach to save souls in this world; Christ and his disciples, to lose them. Which of the two do you choose to follow as master? But he makes himself obvious who argues thus: this harms the eyes, this harms the head, and that harms the chest or the stomach. Certainly each one brings forth into the open what he has learned from his own master. Have you read these distinctions in the Gospel, or in the prophets, or in the letters of the apostles? "Flesh and blood have surely revealed this wisdom to you, not the Spirit of the Father"; for this is "the wisdom of the flesh." But hear what our physicians think of it. "The wisdom of the flesh," they say, "is death"; likewise: "The wisdom of the flesh is hostile to God" (Rom 8:6-7). Ought I to have set before you the opinion of Hippocrates or Galen, or surely from the school of Epicurus? I am a disciple of Christ; I speak to disciples of Christ: if I introduce a foreign doctrine, I myself have sinned. Epicurus and Hippocrates, the one puts forward the pleasure of the body, the other good physical condition; my Master preaches contempt for both. The life of the soul in the body, which the one with utmost zeal inquires whence to sustain, the other whence even to delight, and teaches others to inquire, the Savior counsels even to lose. — Sermons on the Song of Songs, Sermon 30