Doors are an amazing invention, amazing because a door is not a door unless it is able to move. Whether they be screen doors, pocket doors, doors that roll up, doors that slide across, or even doors that fold in two. Doors mark and direct entry into a particular place. Some doors are works of art, beautiful to behold, while other doors are makeshift, dingy and hardly attractive. A door is still a door if it does what doors are supposed to do.
Doors let some things in.
Doors keep some things out. The first door in the Bible is suggested as a shut door. Like a beast of prey, sin was lurking outside the door to Cain's heart. It's even harder not to feel sorrow when we discover that somehow the door was made to open, because Cain used violence to kill his brother. A whole new world where there is no more crying, no more dying, no more darkness and nothing wicked nor unclean.