Rev 3:8 "I know thy works: behold, I have set before thee an open door, and no man can shut it..."
Rev 3:20 "Behold, I stand at the door, and knock..."
The church that has a little strength has before it an open door....the church that thinks it has everything has before it a shut door and will have to "buy of me gold tried in the fire".That door here that no man has shut,is,I believe,the rapture.
And Revelation 3:8 that talks about the open door comes right before Revelation 3:10 where Jesus makes the promise to the church that has not denied Him and has persevered that He will take them off the scene before He sends judgment.