John Gill Commentary Psalms 91:4

"He will cover thee with his pinions, And under his wings shalt thou take refuge: His truth is a shield and a buckler." — Psalms 91:4 (ASV)

He shall cover you with his feathers
As birds do their young, who cannot cover themselves: this they do from a tender regard to them, whereby they both keep them warm, and protect them from those that would hurt them: this represents the helpless state of the children of God, who are, like to young birds, weak and unable to defend themselves: the tender regard of God unto them, as the eagle and other birds have to their young; see (Deuteronomy 32:11) (Isaiah 31:5) (Matthew 23:37) and the warmth and comforts souls have, as well as protection, under his powerful and gracious presence; he comforts them under their tribulations, as well as defends them from their enemies:

and under his wings shall you trust; (See Gill on Psalms 91:1) and the passages there referred to; the same metaphor is continued:

his truth shall be your shield and buckler ;
his faithfulness, which is engaged to keep and preserve his saints safe to his kingdom and glory, (1 Corinthians 1:8 1 Corinthians 1:9) (1 Thessalonians 5:23 1 Thessalonians 5:24) , his Son, who is "truth" itself, (John 14:6) , and whose person, blood, righteousness, and salvation, are as a shield and buckler all around the saints, to secure them from ruin and destruction; and are the shield which faith lays hold on, and makes use of, against the temptation, of Satan; see (Psalms 84:11) (Ephesians 6:16) , the word of God also, which is truth, (John 17:19) , every promise in it, and doctrine of it, is as a shield and buckler to strengthen, support, and secure the faith of his people, (Proverbs 30:5) .