'Open Door' in the Bible

I know thy works; behold, I have set before thee an open door, which no one can shut; for thou hast little power, and yet hast kept my word, and hast not denied my name.

After this I saw, and lo! an open door in heaven; and the former voice, which I heard as of a trumpet speaking with me, said, Come up hither, and I will show thee what things must take place after these.