'Open Door' in the Bible

I know about your deeds. (See, I have placed before you an open door, which no one can close). [Note: This probably refers to their being assured of some unspecified opportunity]. [I know] that you have [only] a little bit of strength, but [in spite of that] you have obeyed my word and did not deny my name.

After [seeing all] these things, I looked, and there in heaven I saw [a vision of] an open door, and the voice I had first heard speaking to me like [the sound of] a trumpet [See 1:10], said, "Come up here [i.e., to heaven], and I will show you the things which must happen after these things [i.e., the things just revealed]."