'Door' in the Bible

"I know your works (behold, I have set before you an open door, which no one can shut), that you have a little power, and kept my word, and didn't deny my name.

Behold, I stand at the door and knock. If anyone hears my voice and opens the door, then I will come in to him, and will dine with him, and he with me.

After these things I looked and saw a door opened in heaven, and the first voice that I heard, like a trumpet speaking with me, was one saying, "Come up here, and I will show you the things which must happen after this."