9 But then, says some one, since the devil is thus cast out of the hearts of believers, does he now tempt none of the faithful? Nay, verily, he does not cease to tempt. But it is one thing to reign within, another to assail from without; for in like manner the best fortified city is sometimes attacked by an enemy without being taken. And if some of his arrows are discharged, and reach us, the apostle reminds us how to render them harmless, when he speaks of the breastplate and the shield of faith. And if he sometimes wounds us, we have the remedy at hand.
For as the combatants are told, “These things I write unto you, that you sin not:” so those who are wounded have the sequel to listen to, “And if any man sin, we have an Advocate with the Father, Jesus Christ, the righteous; and He is the propitiation for our sins.” And what do we pray for when we say, “Forgive us our debts,” but for the healing of our wounds? And what else do we ask, when we say, “Lead us not into temptation,” but that he who thus lies in wait for us, or assails us from without, may fail on every side to effect an entrance, and be unable to overcome us either by fraud or force?
Nevertheless, whatever engines of war he may erect against us, so long as he has no more a place in the heart that faith inhabits, he is cast out. But “except the Lord keep the city, the watchman wakes but in vain.” Presume not, therefore, about yourselves, if you would not have the devil, who has once been cast out, to be recalled within.
Source: Tractates on the Gospel of John (New Advent)