"Withal praying also for us, that God would open unto us a door of utterance, to speak the mystery of Christ, for which I am also in bonds: That I may make it manifest, as I ought to speak" (Colossians 4:3-4).
This was Paul's prayer request of the Colossian Christians, that God would open the door for His testimony. Paul had written earlier about "when I came to Troas to preach Christ's gospel, and a door was opened unto me of the Lord" (2 Corinthians 2:12). The purpose of an open door is thus to preach the gospel of Christ and to speak the mystery of Christ.
Furthermore, these passages indicate that such doors are opened by the Lord, not by human devices. In fact, Christ Himself is "He that openeth, and no man shutteth; and shutteth, and no man openeth" (Revelation 3:7). Doors of testimony are opened by the Lord in answer to prayer, but He also specifies three criteria for keeping the door opened. "I have set before thee an open door, and no man can shut it: for thou hast a little strength, and hast kept my word, and hast not denied my name" (Revelation 3:8).
These conditions mean, literally, having little strength of one's own and thus depending only on God, jealously guarding the integrity of God's Word, and upholding the name of Christ as Creator, Savior, and coming King.
Even when the door is kept open by God, there is no assurance of ease in entering it. Paul wrote that "a great door and effectual is opened unto me, and there are many adversaries" (1 Corinthians 16:9). This is the reason prayer is needed, relying on God, not man!
The Lord is also seeking an open door into churches that think they "have need of nothing. . . . Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him" (Revelation 3:17,20).