7 Attend, in short, to His own words. For just as if we had been inquiring what He meant by saying, “Now is the judgment of the world,” He proceeded to explain it when He says, “Now shall the prince of this world be cast out.” What we have thus heard was the kind of judgment He meant. Not that one, therefore, which is yet to come in the end, when the living and dead shall be judged, some of them set apart on His right hand, and the others on His left; but that judgment by which “the prince of this world shall be cast out.”
In what sense, then, was he within, and whither did He mean that he was to be cast out? Was it this: That he was in the world. and was cast forth beyond its boundaries? For had He been speaking of that judgment which is yet to come in the end, some one's thoughts might have turned to that eternal fire into which the devil is to be cast with his angels, and all who belong to him—that is, not naturally, but through moral delinquency; not because he created or begot them, but because he persuaded and kept hold of them: some one, therefore, might have thought that that eternal fire was outside the world, and that this was the meaning of the words, “he shall be cast out.”
But as He says, “Now is the judgment of this world,” and in explanation of His meaning, adds, “Now shall the prince of this world be cast out,” we are thereby to understand what is now being done, and not what is to be, so long afterwards, at the last day. The Lord, therefore, foretold what He knew, that after His own passion and glorification, many nations throughout the whole world, in whose hearts the devil was an inmate, would become believers, and the devil, when thus renounced by faith, is cast out.
Source: Tractates on the Gospel of John (New Advent)