A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 91:1David is the most probable author; and the pestilence, mentioned in Sa2 24:13-15, the most probable of any special occasion to which the Psalm may refer. The changes of person allowable in poetry are here frequently made. (Psa. 91:1-16)
dwelleth in the secret place-- (Psa 27:5; Psa 31:20) denotes nearness to God. Such as do so abide or lodge secure from assaults, and can well use the terms of trust in Psa 91:2.
psa 91:3snares . . . [and] . . . noisome pestilence--literally, "plagues of mischiefs" (Psa 5:9; Psa 52:7), are expressive figures for various evils.
psa 91:4For the first figure compare Deu 32:11; Mat 23:37.
buckler--literally, "surrounding"--that is, a kind of shield covering all over.
psa 91:5terror--or, what causes it (Pro 20:2).
by night--then aggravated.
arrow--that is, of enemies.
psa 91:7The security is more valuable, as being special, and, therefore, evidently of God; and while ten thousands of the wicked fall, the righteous are in such safety that they only see the calamity.
psa 91:9This exemption from evil is the result of trust in God, who employs angels as ministering spirits (Heb 1:14).
psa 91:13Even the fiercest, strongest, and most insidious animals may be trampled on with impunity.
psa 91:14God Himself speaks (compare Psa 46:10; Psa 75:2-3). All the terms to express safety and peace indicate the most undoubting confidence (compare Psa 18:2; Psa 20:1; Psa 22:5).
set his love--that of the most ardent kind.
psa 91:16show him--literally, "make him see" (Psa 50:23; Luk 2:30).