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 Pauls prayer request of the Colossian Christians, that God would open the door for His testimony. He had written earlier about when I came to Troas to preach Christs gospel, and a door was opened unto me of the Lord (II 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 ones own and thus depending only on God, jealously guarding the integrity of Gods 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 (I Corinthians 16:9). This is the reason why 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). HMM