3 But lest any be sluggish to go forward, let him hear: “Children, it is the last hour.” Go forward, run, grow; “it is the last hour.” This same last hour is long; yet it is the last. For he has put “hour” for “the last time;” because it is in the last times that our Lord Jesus Christ is to come. But some will say, How the last times? How the last hour? Certainly antichrist will first come, and then will come the day of judgment. John perceived these thoughts: lest people should in a manner become secure, and think it was not the last hour because antichrist was to come, he said to them, “And as you have heard that antichrist is to come, now are there come many antichrists.” Could it have many antichrists, except it were “the last hour”?
Source: Homilies on the First Epistle of John (New Advent)