What's meant by "in the symbol of windows and of doors" in "The Dagger with Wings"?

Question

In "The Dagger with Wings" by G. K. Chesterton, the author was describing a situation where Father Brown was sitting in a central room with a staircase leading up from it on one side and a door leading out of it on the other, and central door with red glass let into a passage that ended with second door on the garden.

Then Father Brown went back and sat down again, staring at the dark carpet, which again glowed blood-red with the light from the glass door. Something in the filtered light set his mind drifting on certain borderlands of thought, with the first white daybreak before the coming of colour, and all that mystery which is alternately veiled and revealed in the symbol of windows and of doors.

It seemed that the dark carpet glowed blood-red with the light from the glass door because the second door behind it had been opened and shut on the garden, but I can't get what he meant by "all that mystery which is alternately veiled and revealed in the symbol of windows and of doors"?

Answer 1

Windows and doors are often used as symbols, representing transition, or access to something, especially without fully immersing in it. Consider:

• when one door closes, another opens
• I shut the door on that possibility
• the eyes are the window to the soul
• for a brief window anything seemed possible

The passage seems to me to say that being surrounded by windows and doors got him thinking about transitions, possibilities, glimpses of mystery, and so on.

Answer 2

After a lot of searching and thinking, I think that he mean that Father Brown felt a mystery which is alternately veiled and revealed (as if) some general doors and windows (not necessarily the doors and windows of this room) were opened to reveal it then closed to veil it again.