to knock: to strike a shut door with the hand so that someone inside will open it
This verb names one small action: striking a shut door with the hand so that whoever is inside will come and open it. In wider Greek it covers striking things in general, including striking the strings of an instrument, but in the New Testament it is always a knock at a door, and in nearly every case the writer follows it at once with the word for opening.
What gives the word its force is everything a knock takes for granted. You are outside. The door is shut. You cannot open it yourself, and no amount of knocking opens it. All you can do is make yourself heard and wait for someone with the standing to open. That is why the same plain verb serves both an errand at a courtyard gate and some of the most memorable pictures in the New Testament.
Three uses run through the nine places it appears:
- a caller knocks at a house door and someone inside comes to answer
- knocking pictures asking God persistently, joined to asking (αἰτέω) and seeking (ζητέω) in a threefold command with a promise attached
- the Lord himself stands outside a door and knocks to be let in
The last of these turns the picture around. Almost everywhere a person knocks and God opens. In two scenes it is the Lord who waits outside, and the people indoors are the ones who must open to him.