My question is what is the proper contextual interpretation of
Revelation 3:20: "Behold, I stand at the door, and knock: if any man
hear my voice, and open the door, I will come in to him, and will sup
with him, and he with me"? Some of my friend and I have been discussing
this for awhile. Some say: it's a call for the unsaved world and others
say that in each letter to the churches Jesus offers commendation a
rebuke and an exhortation to repent. Who is He telling to repent here?
Response: There is a detailed study of the seven churches posted at Ichthys which deals with all the specifics of this passage in depth (see the link: The Coming Tribulation: a History of the Apocalypse: Part 2: The Time of the Tribulation and the Signs of its Coming: Subpart A: The Seven Churches of Revelation [Revelation 2:1 - 3:22]). That said, I am happy to give you the "bottom" line on my interpretation of this passage.
First, this is clearly a message from Christ to His Church, since this is part of the message to Laodicea, one of the "seven churches" which represent the Church of Christ militant throughout the entire Church Age. It is certainly true that Jesus calls to all mankind to repent and turn to Him (cf. Matt.4:17), but our Lord's message in this context is to His Church in general, and to the last echelon of that Church in particular, namely, to Laodicea. The call to "open up" is a rebuke, because there is no way that any Christian, let alone an entire portion of the Church, should even think of "closing the door" in His face - but that is the situation with Laodicea (later described as "lukewarm" and about to be "spit out" of Christ's mouth for their disinterest).
In fact, Laodicea represents our own era of the Church, the last one in the series of seven, the last one before the Tribulation. The call to repent from lukewarmness is a call to Christians in general in these last days of the Church Age. From His message to Laodicea, we see clearly that the major trend of our day is apathy where developing a close relationship with our Lord through His Word is concerned. In contrast to the previous era (Philadelphia - cf. the great theology and missions of the 18-19th centuries) we suffer by comparison even on the face of things, and it certainly is in keeping with my own experience to say that when one scratches the surface of individuals/organizations of our own day, even those that are putting up a good front, there is really not much content beyond that surface appearance. You are absolutely correct that this message is one to true Christians who are not being as true to Christ as they should be - not out of gross sin in particular, but because of a real lack of interest in Him, in His Word.
So while - certainly - God wants all to repent and come to Him, this admonition from our Lord is directed to us, the last Church era of Laodicea (as in all the other church messages the commands and observations apply primarily to them), and so by application the admonition applies to all Christians of our present day whose attitude to Him is lukewarm (sadly the norm we see at present, and one which scripture suggests will intensify as time passes).
This sorry trend, by the way, will put those so affected in a very weak position once the Tribulation begins. See "The Great Apostasy" in Coming Tribulation: Part 3A: From the Seventh Seal to the Two Witnesses.
You might also have a look at these links:
Mega-Churches, Emergent Christianity, Spirituality and Materialism.
Church: The Biblical Ideal versus the Contemporary Reality.
Hope this is of some help with your question - please don't hesitate to write me back if it's not completely clear.
Yours in the One who died for us that we might rejoice in Him forever in eternal life, our Lord and Savior Jesus Christ.