'Door' in the Bible

‘I know your deeds. Behold, I have put before you an open door which no one can shut, because you have a little power, and have kept My word, and have not denied My name.

Behold, I stand at the door and knock; if anyone hears My voice and opens the door, I will come in to him and will dine with him, and he with Me.

After these things I looked, and behold, a door standing open in heaven, and the first voice which I had heard, like the sound of a trumpet speaking with me, said, “Come up here, and I will show you what must take place after these things.”