Oecumenius

It is not that there is literally a door in heaven that opens and closes at times, but it was shown to the evangelist in this way so that he might see the things above the heavens. For with every door opened, what lies within must inevitably be seen. And I heard a voice, which sounded like a trumpet, saying these words to me: Come up here so that you may see what is to come.

Commentary on Revelation ↗

Bede

After this, I looked, and behold, a door open in heaven. After describing the works of the Church, what it was and what it would be like, he recapitulates from the birth of Christ, intending to say the same things differently. For he repeats the entire time of the Church in various figures in this book. Behold, he says, a door open in heaven. Fittingly, he sees a heavenly door about to ascend, to whom high mysteries are promised to be revealed. …

Commentary on Revelation ↗

Alcuin of York

After these things I looked, and behold a door was opened in heaven. The Redeemer's Nativity is recapitulated, and these things had already been said with various other symbols; as when one relates one event in different ways, the narrations will each have a different time, but not the event itself, which happened in one time. We should however ask the question of why John does not disclose the time of his prophecy, while other prophets used to disclose by the kings the length of the interval between one vision and another. …

Commentary On Revelation ↗

Victorinus of Pettau

"After this, I beheld, and, lo, a door was opened in heaven." The new testament is announced as an open door in heaven. "And the first voice which I heard was, as it were, of a trumpet talking with me, saying, Come up hither." Since the door is shown to be opened, it is manifest that previously it had been closed to men. And it was sufficiently and fully laid open when Christ ascended with His body to the Father into heaven. …

Commentary on the Apocalypse of the Blessed John ↗