38 Then Saul gave David his own armor-a bronze helmet and a coat of mail. 39 David put it on, strapped the sword over it, and took a step or two to see what it was like, for he had never worn such things before. "I can't go in these," he protested to Saul. "I'm not used to them." So David took them off again.
Matthew Henry's Commentary on 1 Samuel 17:38-39
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.