Theophylact of Ohrid

The Lord commands this to all in general — both to people in the world and to hermits. Therefore, we must watch and pray, fulfilling both the one and the other, for many, although they watch, spend the nights not in prayer but in wicked deeds. Note also that Christ did not now say, "I do not know when the time of the end will come," but "you do not know," for He hid this time for our benefit. If even in the uncertainty of the end we are hostile toward one another, what would we not have done if we knew the time of the end? …

Commentary on Mark ↗

Augustine of Hippo

For He not only speaks to those in whose hearing He then spake, but even to all who came after them, before our time, and even to us, and to all after us, even to His last coming. But shall that day find all living, or will any man say that He speaks also to the dead, when He says, Watch, lest when he cometh he find you sleeping? Why then does He say to all, what only belongs to those who shall then be alive, if it be not that it belongs to all, as I have said? …

Catena Aurea by Aquinas ↗

Bede

But what I say to you, I say to all: Keep watch. Not only the apostles and their successors, indeed the leaders of the Church, but all are commanded to keep watch. We are all urgently ordered to guard the doors of our hearts, lest the ancient enemy break in by suggesting evil. We must each carefully avoid being found sleeping when the Lord comes. For each of us will give an account to God. He keeps watch who has the eyes of his mind open to the vision of true light. He keeps watch who, by working, keeps what he believes. …

On the Gospel of Mark ↗