Oecumenius

Behold, it says, I stand at the door and knock; if anyone hears my voice and opens the door, I will come in to him and dine with him, and he with me. The Lord demonstrates to His own as the gentle and peaceful One. For the Devil strikes with "an axe and a chisel" the doors of those who do not receive him, according to the voice of the prophet (Ps. 73:6). But the Lord even now, and in the Songs, says to the bride, "Open to me, my sister, my bride." (Song 5:2) And if anyone opens to Him, He enters. …

Commentary on Revelation ↗

Jerome

To the terrible one among the kings of the earth. This is understood also according to the letter. But may God grant to us also that we may be kings of the earth: kings of the earth, that we may rule over our flesh. Whence the apostle also says: "Let not sin reign in your mortal body." And in another place it is written: "The heart of the king is in the hand of God." … Kings, then, are the saints, and the heart of these is in the hand of the Lord. …

Homily 9, on Psalm 75 ↗

Bede

The Lord stands at the doorway and knocks when he pours into our heart the memory of his will, either through the mouth of a man who is teaching [us] or through his own internal inspiration. When his voice is heard, we open the gate to receive [him] when we willingly present our assent to [his] counsels, whether secret or open, and devote ourselves to accomplishing those things that we recognize are to be done. He comes in order to eat with us and we with him. …

Homilies on the Gospels 1.21 ↗

Alcuin of York

Behold, I stand at the gate, and knock. If any man shall hear my voice, and open to me the door, I will come in to him, and will sup with him, and he with me. This is not to be taken as a physical act of standing, but as a placeless one, because when the Lord dwells in someone, he does not leave him to pass to someone else. …

Commentary On Revelation ↗