It has a map of Jerusalem on the back of the door, to remind me that, when I go through that door, I go to worship with the New Jerusalem, God's own people. I have a picture to place above it from the book Where the Wild Things Are , to remind me that some of God's own people aren't that tame yet, so I shouldn't stick my fingers through the cage. We have to look at it, not through them, when we look at the painting. I think she is pointing to the truth that all true faith is a doorway to another reality. And these doors are essential, not just a small part of reality.
Doors are barriers between inside and outside, danger and safety.
Many of us love to cozy up right where we are and shut the door. Like the first disciples, we like our safety. But often there is more danger in not opening a door than there is in opening one. Coziness can be wonderful, but God promises us movement. With God we get adventure, not safety.