Often taken out of context verse 20 is used to present the offer of the gospel to all who accept it. I remember my days with Campus Crusade for Christ, now with Christ dropped from the name and just called CRU. It was a goto witnessing verse.
Perhaps because of the narrow way of salvation, and the reality of God’s effectual call it is more likely a statement of fact to the church that is willing to actually see the signs of his coming. There is a tone of assurace here, but the message is neither one broadly accepted, nor possible to accept without the inner working of God’s Spirit on one’s heart…..the real question then is found in verse 23…will any of the churches hear?
Rev 3:20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.
Rev 3:21 To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne.
Rev 3:22 He that hath an ear, let him hear what the Spirit saith unto the churches.
Here is old John Gill’s comment.
Revelation 3:20 Gill
Behold, I stand at the door and knock,…. The phrase of standing at the door may be expressive of the near approach, or sudden coming of Christ to judgment, see Jas 5:9; and his knocking may signify the notice that will be given of it, by some of the immediate forerunners and signs of his coming; which yet will be observed but by a few, such a general sleepiness will have seized all professors of religion; and particularly may intend the midnight cry, which will, in its issue, rouse them all:
if any man hear my voice; in the appearances of things and providences in the world:
and open the door; or show a readiness for the coming of Christ, look and wait for it, and be like such that will receive him with a welcome:
I will come unto him, and sup with him, and he with me; to and among these will Christ appear when he comes in person; and these being like wise virgins, ready, having his grace in their hearts, and his righteousness upon them, he will take them at once into the marriage chamber, and shut the door upon the rest; when they shall enjoy a thousand years communion with him in person here on earth; when the Lamb on the throne will feed them with the fruit of the tree of life, and lead them to fountains of living water, and his tabernacle shall be among them.