Augustine, City of God, book 20, chapter 8:
“After this,” says St John, “he must be unbound for a little while” (Rev 20:3). If the devil is bound and locked up, so that he cannot seduce the Church, will his deliverance consist in having this power [of seducing her]? God forbid! He will never seduce the Church predestined and chosen before the creation of the world, of which it is said that “The Lord knows those who are his” (2 Tim 2:19). However, there will be a Church here below, in the time when the devil must be unbound, just as there has always been one since Jesus Christ. A little later, Saint John says that the devil, once unbound, will lead the nations he has seduced throughout the world to make war on the Church, and that the number of his enemies will equal the sands of the sea: “And they spread out,” he says, “over the earth, and surrounded the camp of the saints and the beloved City of God. But God sent down fire from heaven, which devoured them; and the devil, who deceived them, was cast into a lake of fire and brimstone with the beast and the false prophet, to be tormented there day and night for ever and ever” (Rev 20:8-10). This passage concerns the Last Judgment, and yet I have been pleased to report it, lest it be imagined that, in the short time that the devil is to be unbound, there will be no Church in this world, either because he no longer finds it there, or because he destroys it by his persecutions. So the devil has not been bound for the whole of the time covered by Revelation, from the first advent of Jesus Christ to the end of the world, when the second will take place. And this is what Saint John calls a thousand years, so that Scripture means that the devil will not seduce the Church during this interval, since he will not seduce her either when he is unbound. Indeed, it is indubitable that if being bound is for him to be unable to seduce the Church, he will be able to do so when he is unbound. To be bound in relation to the devil is therefore not to have permission to tempt men as much as he can, by skill or violence, to make them go over to his side. If he were allowed to do this for such a long time, the weakness of men is such that he would cause a large number of the faithful to fall away, and prevent many men from becoming faithful, which is something God does not want. So it was to prevent this that he bound him.
But he’ll be unbound when there will be only a short time left. Scripture tells us that the devil and his accomplices will turn all their rage against the Church for three and a half years; and those with whom he will have to deal will be such that he will not be able to overcome them either by force or by artifice. Now, if he were never unbound, we would not know so well his power and malignity, nor the patience of the holy city, nor the admirable wisdom with which the Almighty has known how to make use of the devil’s malice, either by not preventing him from seducing the saints, in order to exercise their virtue, or by not allowing him to use all his fury, lest he triumph over an infinite number of weak men who were to swell the ranks of the Church. He will therefore be unleashed at the end of time, so that the City of God may recognize, to the glory of its Redeemer and Liberator, what an adversary it has overcome. What are we in comparison with the Christians who will then be, since they will be overcoming a raging enemy, which we have great difficulty in combating, bound as it is? Nevertheless, there is no doubt that during this very interval, God has had and still has soldiers so brave and experienced that, even if they were alive when the devil is unbound, they would fear neither his efforts nor his wiles.
Now, the devil was not only bound when the Church began to spread from Judea among the nations; he is bound even now, and will be bound until the end of the centuries, when he must be unbound. We still see people every day leaving their infidelity, in which the devil held them, and embracing the faith; and there will always be some who will be converted until the end of the world. The strong one is bound in the same way with regard to each of the faithful, when they are taken away from him as his prey; as, on the other hand, the abyss in which he was locked up was not destroyed by the death of the first persecutors of the Church; but to these others have succeeded and will succeed them to the end of the centuries, so that he is always locked up in these hearts full of passion and blindness, as in a deep abyss. Now, it is a question of knowing whether, during these last three and a half years when the devil will exercise all his fury, there will still be a few men, among the faithful, who will embrace the faith. How would this word be justified: “No one can enter a strong man’s house and take away his possessions, unless he first binds him” (Mt 12:29), if they are taken away even when he is unbound? It would seem, then, that this forces us to believe that in this short time the Church will make no new conquests, but that the devil will fight only against those who are already Christians; and if some of these are defeated, it must be said that they were not among those predestined. It is not in vain that the same Saint John, who wrote the Apocalypse, said of some of them in one of his Epistles: “They came out from among us, but they were not of us; for if they had been of us, they would have remained there” (Jn 2:19). But what can we say about little children? It is unbelievable that this latest persecution will not find any unbaptized Christians, or that they will not even be born during this time, and if they are, that their parents will not baptize them. How, then, are these goods to be taken away from Satan, since he will be unbound, and, as the Lord says: “No one enters his house and takes away his goods, unless he has first bound him”? So let us rather believe that, even during this time, apostasies will not be lacking, nor conversions, and that parents will have enough courage to baptize their children, as well as new converts, that they will overcome this strong man, however unbound he may be, that is to say, even though he employs against them tricks and maneuvers that he has not yet used, nonetheless, they will still be taken from him, even though he is not bound. Nevertheless, the words of the Gospel will always stand: “No one can enter the house of the strong man, nor take away his goods, unless he has first bound him”. This order was indeed obeyed. First the strong man was bound, and then his possessions were taken from all nations, to make up the Church, which has since grown and strengthened to the point of being able to strip the devil, even when he is unbound. Just as it must be admitted that the charity of many will grow cold, because crime will be triumphant s, and that many, who are not written in the book of life, will succumb under the unheard-of persecutions of the devil already unbound, so it must be believed that not only true Christians, but that some of those outside the Church, aided by the grace of God and the authority of the Scriptures, which have foretold the end of the world they will see coming, will be more willing to believe what they did not believe, and stronger to defeat the devil, unleashed as he will be. Let us say, in this state of affairs, that he was bound so that his possessions could be taken away from him, even though he will be unbound, according to the Savior’s words: “How can one enter the house of the strong to take away his possessions, unless he has been bound first?”