'Door' in the Bible

"To the messenger of the church in Philadelphia, write: "The one who is holy, who is true, who has the key of David, who opens a door that no one can shut, and who shuts a door that no one can open, "says this:

"I know what you've been doing. Look! I have put in front of you an open door that no one can shut. You have only a little strength, but you have obeyed my word and have not denied my name.

Look! I am standing at the door and knocking. If anyone listens to my voice and opens the door, I will come in to him and eat with him, and he will eat with me.

After these things, I saw a door standing open in heaven. The first voice that I had heard speaking to me like a trumpet said, "Come up here, and I will show you what must happen after this."