1 Peter 5:8

8Be sober This explanation extends wider, that as we have war with a most fierce and most powerful enemy, we are to be strenuous in resisting him. But he uses a twofold metaphor, that they were to be sober, and that they were to exercise watchfulness. Surfeiting produces sloth and sleep; even so they who indulge in earthly cares and pleasures, think of nothing else, being under the power of spiritual lethargy.

We now perceive what the meaning of the Apostle is. We must, he says, carry on a warfare in this world; and he reminds us that we have to do with no common enemy, but one who, like a lion, runs here and there, ready to devour. He hence concludes that we ought carefully to watch. Paul stimulates us with the same argument in Ephesians 6:10, where he says that we have a contest not with flesh and blood, but with spiritual wickedness, etc. But we too often turn peace into sloth, and hence it comes that the enemy then circumvents and overwhelms us; for, as though placed beyond the reach of danger, we indulge ourselves according to the will of the flesh.

He compares the devil to a lion, as though he had said, that he is a savage wild beast. He says that he goes round to devour, in order to rouse us to wariness. He calls him the adversary of the godly, that they might know that they worship God and profess faith in Christ on this condition, that they are to have continual war with the devil, for he does not spare the members who fights with the head.