Rediscovered this today in the Institutes:
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.
Calvin, Institutes, 1.14.13.