This letter is addressed to the church in Laodicea. A church is a gathering of true Christian believers (in the Bible a church is not a building, so the door mentioned here is not a literal door). The term “door” is a metaphor for access to the church. The church in Laodicea is lukewarm, repulsive to Jesus so that he is about to spit them out (Revelation 3:16), and so Jesus stands outside the gathering. With their brackish taste in his mouth he yet in love (Revelation 3:19) knocks on their door to call for repentance before he would spit them out.
20 Behold, I stand at the door and knock. If anyone hears my voice and opens the door, I will come in to him and eat with him, and he with me.