First, let’s look at the entire passage in context (the verse quoted is in bold):
"And to the angel of the church in Laodicea write: 'The words of the Amen, the faithful and true witness, the beginning of God’s creation.
I know your works: you are neither cold nor hot. Would that you were either cold or hot! So, because you are lukewarm, and neither hot nor cold, I will spit you out of my mouth. For you say, I am rich, I have prospered, and I need nothing, not realizing that you are wretched, pitiable, poor, blind, and naked. I counsel you to buy from me gold refined by fire, so that you may be rich, and white garments so that you may clothe yourself and the shame of your nakedness may not be seen, and salve to anoint your eyes, so that you may see. Those whom I love, I reprove and discipline, so be zealous and repent.
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. The one who conquers, I will grant him to sit with me on my throne, as I also conquered and sat down with my Father on his throne. He who has an ear, let him hear what the Spirit says to the churches.’" [Rev 3:14-22; ESV]
Under the dictation of the Lord, the apostle John is writing Christ’s warnings and encouragements to the seven churches. This passage is addressed to “the church in Laodicea” (v. 14). He warns that He finds them lukewarm, neither cold nor hot (v. 15-16), and likewise claim to be wealthy and in no need of anything, when in fact they are poor and wretched (v. 17). This latter point is entirely spiritual in context, for He tells them to seek from Him “gold refined by fire” and “rich, and white garments so that you may clothe yourself” (v. 18).
Speaking from this same context of despising lukewarm faith and seeking from God true wealth and clothing, Christ says: “Those whom I love, I reprove and discipline, so be zealous and repent” (v. 19). After this request, He states: “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” (v. 20). He adds an additional promise to this: “The one who conquers, I will grant him to sit with me on my throne, as I also conquered and sat down with my Father on his throne” (v. 21). Finally, He concludes this as He does to all the churches: “He who has an ear, let him hear what the Spirit says to the churches” (v. 22), again establishing that the audience being spoken to are the churches.
What does this passage mean?
- Christ is speaking to the Christians who have yet to be judged. Unlike the popular belief that this is a general statement to everyone, this is a statement to believers and supposed believers. At least two verses illustrate specifically that this is addressed to the church (verses 14 and 22).
- The immediate concern is for those who are “lukewarm,” and thus carry a weak faith that is neither for God or against them. They likewise believe themselves to be self-sufficient, and essentially believe that they have no need for God.
- He commands them to be zealous repent, for He is coming. Those are attentive and ready will open the door when He knocks, and thus He will enter and eat with them (v. 20), and they will be glorified with Him and the Father (v. 21). Hence on the return of Christ, they will find themselves a Day of Eternal Life, as opposed to a Day of Judgment for those not ready and unbelieving.
To summarize: the verse is a warning to believers to be ready for the arrival of Christ.
The “other sheep” our Lord was referring to were the Gentiles, who were to be brought into the fold. You are right, however, that he brings them into the herd - God is the active party here, not the sheep. Sheep are not known for picking and choosing their shepherds.