31 Now when the words which David spoke were heard, they reported them to Saul; and he sent for him. 32 Then David said to Saul, "Let no man's heart fail because of him; your servant will go and fight with this Philistine." 33 And Saul said to David, "You are not able to go against this Philistine to fight with him; for you are a youth, and he a man of war from his youth." 34 But David said to Saul, "Your servant used to keep his father's sheep, and when a lion or a bear came and took a lamb out of the flock, 35 I went out after it and struck it, and delivered the lamb from its mouth; and when it arose against me, I caught it by its beard, and struck and killed it. 36 Your servant has killed both lion and bear; and this uncircumcised Philistine will be like one of them, seeing he has defied the armies of the living God." 37 Moreover David said, "The Lord, who delivered me from the paw of the lion and from the paw of the bear, He will deliver me from the hand of this Philistine." And Saul said to David, "Go, and the Lord be with you!" 38 So Saul clothed David with his armor, and he put a bronze helmet on his head; he also clothed him with a coat of mail. 39 David fastened his sword to his armor and tried to walk, for he had not tested them. And David said to Saul, "I cannot walk with these, for I have not tested them." So David took them off.
Matthew Henry's Commentary on 1 Samuel 17:31-39
Commentary on 1 Samuel 17:31-39
(Read 1 Samuel 17:31-39)
A shepherd lad, come the same morning from keeping sheep, had more courage than all the mighty men of Israel. Thus God often sends good words to his Israel, and does great things for them, by the weak and foolish things of the world. As he had answered his brother's passion with meekness, so David answered Saul's fear with faith. When David kept sheep, he proved himself very careful and tender of his flock. This reminds us of Christ, the good Shepherd, who not only ventured, but laid down his life for the sheep. Our experience ought to encourage us to trust in God, and be bold in the way of duty. He that has delivered, does and will continue to do so. David gained leave to fight the Philistine. Not being used to such armour as Saul put upon him, he was not satisfied to go in that manner; this was from the Lord, that it might more plainly appear he fought and conquered in faith, and that the victory was from Him who works by the feeblest and most despised means and instruments. It is not to be inquired how excellent any thing is, but how proper. Let Saul's coat be ever so rich, and his armour ever so strong, what is David the better if they fit him not? But faith, prayer, truth, and righteousness; the whole armour of God, and the mind that was in Christ; are equally needful for all the servants of the Lord, whatever may be their work.