'Door' in the Bible

I know your doings. I have put an opened door in front of you, which no one can shut; because you have but a little power, and yet you have guarded My word and have not disowned Me.

I am now standing at the door and am knocking. If any one listens to My voice and opens the door, I will go in to be with him and will feast with him, and he shall feast with Me.

After all this I looked and saw a door in Heaven standing open, and the voice that I had previously heard, which resembled the blast of a trumpet, again spoke to me and said, "Come up here, and I will show you things which are to happen in the future."

Then I saw a door open in Heaven, and a white horse appeared. Its rider was named "Faithful and True" --being One who in righteousness acts as Judge, and makes war.