John Calvin Commentary 1 Thessalonians 5:8

"But let us, since we are of the day, be sober, putting on the breastplate of faith and love; and for a helmet, the hope of salvation." — 1 Thessalonians 5:8 (ASV)

Having put on the breastplate. He adds this so that he may more effectively shake us out of our complacency, for he calls us, so to speak, to arms, to show that it is not a time to sleep. It is true that he does not use the term war; but when he arms us with a breastplate and a helmet, he admonishes us that we must engage in warfare.

Therefore, whoever is afraid of being surprised by the enemy must keep awake to be constantly on watch. Just as he has exhorted us to vigilance because the doctrine of the gospel is like the light of day, so he now stirs us up with another argument: that we must wage war with our enemy. From this it follows that idleness is too hazardous. For we see that soldiers, though they may be intemperate in other situations, nevertheless, when the enemy is near, refrain from gluttony and all bodily pleasures out of fear of destruction, and are diligently on watch to be on their guard. Therefore, since Satan is on the alert against us and tries a thousand schemes, we ought at least to be no less diligent and watchful.

It is, however, in vain that some seek a more refined exposition of the names of the kinds of armor, for Paul speaks here in a different way from what he does in Ephesians 6:14, where he makes righteousness the breastplate. This, therefore, will suffice for understanding his meaning: that he intends to teach that the life of Christians is like a perpetual warfare, since Satan does not cease to trouble and harass them.

He would have us, therefore, be diligently prepared and on the alert for resistance. Furthermore, he admonishes us that we need arms, because unless we are well armed, we cannot withstand so powerful an enemy. He does not, however, enumerate all the parts of armor, (πανοπλίαν), but simply mentions two: the breastplate and the helmet. Meanwhile, he omits nothing of what belongs to spiritual armor, for the one who is provided with faith, love, and hope will in no way be found unarmed.