And David put his hand in his bag, and took there a stone, and slung it, and struck the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth.
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Ambrose of Milan
David never waged war unless he was driven to it. Thus prudence was combined in him with fortitude in the battle. For even when about to fight single-handed against Goliath, the enormous giant, he rejected the armor with which he was laden. His strength depended more on his own arm than on the weapons of others. Then, at a distance, to get a stronger throw, with one cast of a stone, he killed his enemy. - "Duties of the Clergy 1.35.177"
Forehead. "The soul. More probably resides in the callous body of the brain "(Eyre, Thesis 1797,) between the eyes. (Haydock)
Earth, quite lifeless, (Salien) or unable to resist. (Menochius)
The Balearic slingers scarcely ever missed their mark. (Livy, viii. 4.) The Chaldean supposes that David hit the eye, which was not covered with brass: but the stone might penetrate or kill Goliath through his helmet. Even a buckler is not capable of withstanding their violence. (Diodorus, v. 207.) See Judges xx. 16. (Calmet)
Pride sits on the forehead, and manifests itself by impudent behaviour. We must destroy it by humility, and by the cross of Christ. (St. Augustine) (Worthington)
Therefore, let us take in our hands that stone, I mean the cornerstone, the spiritual rock. If Paul could think in these terms of the rock in the desert, no one will in any way feel resentment against me if I understand David’s stone in the same sense. In the case of the Jews in the desert, it was not the nature of the visible stone but the power of the spiritual stone which sent forth those streams of water. So, too, in David’s case, it was not the visible stone but the spiritual stone which sank into the barbarian’s head. This is why, at that time, I promised that I would say nothing based on rational arguments. “Our weapons are not merely carnal but spiritual, demolishing sophistries and reasoning and every proud height that raises itself against the knowledge of God.” - "Against the Anomoeans 11.6"
Therefore, brothers, let us arm ourselves with heavenly weapons for the coming judgment of the world: let us gird on the breastplate of faith, protect ourselves with the helmet of salvation, and defend ourselves with the word of God as with a spiritual sword. For the one who is arrayed with these weapons does not fear present disturbance and is not afraid of future judgment, since holy David, protected with this devotion, killed the very strong and armed Goliath without weapons and struck down the warlike man, girt about with defenses on all sides, by the strength of his faith alone. For although holy David did not put on a helmet, strap on a shield, or use a lance, he killed Goliath. He killed him, however, not with an iron spear but with a spiritual sword, for although he appeared weaponless in the eyes of human beings, yet he was adequately armed with divine grace. But the spiritual sword itself was not a sword, since it was not by the sword but by a stone that Goliath died when he ...
Having trust in Christ, consigning everything to the God of powers, regarding God alone as all that is highest—this has always been efficacious in achieving every good. This is the faith that has prevailed over all weapons. This was the strength that made that slight boy great, for he grew stronger by spurning weapons and brought low the armed giant by the power of a stone. - "Poems 26.150"