Recently our door to the outside world gave up its mission. It stopped. It had opened and shut so many times it got tired of opening and shutting and its lock stayed securely locked.
The knob didn't work and the key didn't work so both the key and the door were useless, ridiculous and superfluous. They were redundant, no use at all to we who lived inside the house and no use to any neighbour who might want to come to our aid.
Now, as I understand it, a door is the way into a house. If there is no door you can go round and round and over and under but you cannot get inside the house.
You must have a door.
In fact, if you want to get inside anything, you firstly have to find the door. For instance, the alphabet begins with the first letter and if you never find the first letter, you never find the key to open the door of the alphabet.
Likewise, the door to mathematics is a one and you cannot get a two without a one so the door to a two is a one. Equally, the door to a book is the first chapter and if you want the key to open the book and get inside, you have to begin with the first chapter.
Jesus talks about a key and a door. He condemned the lawyers because, He said, "you have taken away the key to knowledge." (Luke 11:52). If you value knowledge, you don't throw its key away but they had, so they could not understand Jesus. They never understood Him because they had taken away the key to the Book that was all about Him.
When Jesus talks about a door, He says, "I stand at the door and knock." (Revelation 3:20).
The door is shut and the key has been taken away. If I were looking
for the key to open the front door of the Bible, I would look in the
first chapter of the Book. I would want to get inside the Book to know
why it was written and to get to know its principal character. Wouldn't
you? After all, doors are for opening.