Jamieson, Fausset and Brown on 1 Thessalonians 5:8

8. Faith, hope, and love, are the three pre-eminent graces (1Th 1:3; 1Co 13:13). We must not only be awake and sober, but also armed; not only watchful, but also guarded. The armor here is only defensive; in Ep 6:13-17, also offensive. Here, therefore, the reference is to the Christian means of being guarded against being surprised by the day of the Lord as a thief in the night. The helmet and breastplate defend the two vital parts, the head and the heart respectively. "With head and heart right, the whole man is right" [Edmunds]. The head needs to be kept from error, the heart from sin. For "the breastplate of righteousness," Ep 6:14, we have here "the breastplate of faith and love"; for the righteousness which is imputed to man for justification, is "faith working by love" (Ro 4:3, 22-24; Ga 5:6). "Faith," as the motive within, and "love," exhibited in outward acts, constitute the perfection of righteousness. In Ep 6:17 the helmet is "salvation"; here, "the hope of salvation." In one aspect "salvation" is a present possession (Joh 3:36; 5:24; 1Jo 5:13); in another, it is a matter of "hope" (Ro 8:24, 25). Our Head primarily wore the "breastplate of righteousness" and "helmet of salvation," that we might, by union with Him, receive both.