'Knocking' in the Bible

“So I say to you, ask and keep on asking, and it will be given to you; seek and keep on seeking, and you will find; knock and keep on knocking, and the door will be opened to you.

For everyone who keeps on asking [persistently], receives; and he who keeps on seeking [persistently], finds; and to him who keeps on knocking [persistently], the door will be opened.