Oecumenius

He says, I know your works, that you are godly; and I have set before you an open door. And he said beforehand: The one who opens, and no one shuts, so also the door which I have given opened, and no one can shut it. Because, he says, you have a little power, and have kept My word, and have not denied My name. Philadelphia is a small city, therefore its strength is also small. But in guarding the faith of Christ beyond her own strength, she rose as unyielding, to remain among those who trouble the faithful. …

Commentary on Revelation ↗

Bede

Because you have a little strength, etc. He shows the reason that this church deserves these gifts, because it does not trust in its own strength but in the grace of Christ the King. It is the praise of the protecting God and the devotion of the Church that the door of victory is opened to one of little faith, and that little strength is strengthened by faith.

Commentary on Revelation ↗

Bonaventure

And it was said to the Angel of Philadelphia, who was the sixth, "Thus says the holy one, the true one, he who has the key of David, he who opens and no one shuts, and who shuts and no one opens: I know thy works. Behold, I have caused a door to be opened before thee." And he said that now for the first time the understanding of Scripture would be given, either as revelation or as the key of David, to a single person or to a great number of people; and I believe rather that it would be given to a great number.

Collations on the Hexaemeron, Collation 16 ↗

Victorinus of Pettau

The sixth class is the mode of life of the best election. The habit of saints is set forth; of those, to wit, who are lowly in the world, and unskilled in the Scriptures, and who hold the faith immoveably, and are not at all broken down by any chance, or withdrawn from the faith by any fear. Therefore He says to them:- "I have set before thee an open door, because thou hast kept the word of my patience." In such little strength.

Commentary on the Apocalypse of the Blessed John ↗