The Victorian-era English artist William Holman Hunt, one of the founders of the Pre-Raphaelite school, is perhaps best known for his 1853 painting, “The Light of the World.” It's a literal depiction of this verse from Revelation. Jesus is literally standing at a door, knocking.
There's one peculiarity about the painting, though, that may appear, at first glance, to be an error on the artist's part. There's no doorknob.
The lack of a doorknob is intentional. For, as the artist himself used to explain to those who asked, Jesus is waiting for us to open the door of our hearts and welcome him in.
There's only one doorknob, and it's on the inside. When Jesus comes knocking, it's up to us to respond.