Adam Clarke on Psalms 91:4

He shall cover thee with his feathers - He shall act towards thee as the hen does to her brood, - take thee under his wings when birds of prey appear, and also shelter thee from chilling blasts. This is a frequent metaphor in the sacred writings; see Psa 17:8 (note), Psa 57:1 (note), Psa 61:4 (note), and the notes on them. The Septuagint has Εν τοις μεταφρενοις αυτου επισκιασει σοι· He will overshadow thee between his shoulders; alluding to the custom of parents carrying their weak or sick children on their backs, and having them covered even there with a mantle. Thus the Lord is represented carrying the Israelites in the wilderness. See Deu 32:11-12 (note), where the metaphor is taken from the eagle.

His truth shall be thy shield and buckler - His revelation; his Bible. That truth contains promises for all times and circumstances; and these will be invariably fulfilled to him that trusts in the Lord. The fulfillment of a promise relative to defense and support is to the soul what the best shield is to the body.