Revelation 3:20

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 ( James 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.