John Calvin on Spiritual Warfare

All that Scripture teaches concerning devils aims at arousing us to take precaution against their stratagems and contrivances, and also to make us equip ourselves with those weapons which are strong and powerful enough to vanquish these most powerful foes.

For when Satan is called the god [II Cor. 4:4] and prince [John 12:31] of this world, when he is spoken of as a strong armed man [Luke 11:21; cf. Matt. 12:29], the spirit who holds power over the air [Eph. 2:2], a roaring lion [I Peter 5:8], these descriptions serve only to make us more cautious and watchful, and thus more prepared to take up the struggle.

This also sometimes is noted explicitly: for Peter, after he has said that the devil "prowls around like a roaring lion seeking someone to devour" [I Peter 5:8], immediately subjoins the exhortation that with faith we steadfastly resist him [I Peter 5:9]. And Paul, after he has warned us that our struggle is not with flesh and blood, but with the princes of the air, with the powers of darkness, and spiritual wickedness [Eph. 6:12], forthwith bids us put on that armor capable of sustaining so great and dangerous a contest [Eph. 6:13ff].

We have been forewarned that an enemy relentlessly threatens us, an enemy who is the very embodiment of rash boldness, of military prowess, of crafty wiles, of untiring zeal and haste, of every conceivable weapon and of skill in the science of warfare.
We must, then, bend our every effort to this goal: that we should not let ourselves be overwhelmed by carelessness or faintheartedness, but on the contrary, with courage rekindled stand our ground in combat. Since this military service ends only at death, let us urge ourselves to perseverance.

Indeed, conscious of our weakness and ignorance, let us especially call upon God's help, relying upon him alone in whatever we attempt, since it is he alone who can supply us with counsel and strength, courage and armor.

--John Calvin, Institutes of the Christian Religion, 1.15.13. (Philadelphia: Westminster John Knox Press, 2011 edition), pp. 172-173.