God uses metaphors in Scripture as a figure of speech to communicate His truth. In some prominent passages, New Testament authors use the "flock" analogy as one of these pictures to portray a particular theological concept. "Flock," of course, starts with an agrarian reality with actual sheep, as seen in the nativity text in Luke 2:8, "And there were in the same country shepherds abiding in the field, keeping watch over their flock by night." To convey doctrine using a metaphor, we start with the reality, the actual flock led by a real shepherd in a tangible field.
We determine what "flock" means by its usage. In a metaphorical way, the word "flock" (poimnion) is used six times (Mt 26:31; Lk 12:32; Acts 20:28-29; 1 Pet 5:2-3). In each case, like a real flock of sheep, "flock" is a visible group in one locale under a shepherd. It does not illustrate anything universal at all. If someone wanted to communicate a universal concept, he would not use "flock" to do so.
In Matthew 26:31, Jesus speaks to His disciples about the night of His betrayal and quotes Zechariah 13:7 to connect this event as a fulfillment of prophecy. The Shepherd, Jesus, would be smitten, and the sheep, the disciples gathered there, would be scattered. They were an assembled group that would become unassembled. Even if we were to look at this Matthew reference as somewhat ambiguous, which it does not seem to be, we have other references to help us understand how the New Testament uses and how we should understand "flock."
Next, in the gospels one more time, Luke 10:32, Jesus again addresses His disciples as "little flock." Again we see gathered, visible, and local. We have the Shepherd, Jesus, and His flock with Him there in one place. It has all the aspects fitting of the metaphor.
In Acts 20:28-29 we are beginning to become as clear as ever. There Paul is speaking to individual pastors leading an individual church. These were not men in charge of all believers in the world. They were men who were shepherding in one location the believers that assembled there. The wolves would enter into their individual flock, and hurt the sheep that were gathered with them. They did not pastor all the sheep in the world, just the ones in their flock. A flock is again a separate church.
Acts 20:28-29, unambiguous, plain, matches up with 1 Peter 5:2-3, the final usage of "flock" in the New Testament. Peter commands pastors to "feed the flock which is among you." This is to say that there are several flocks. Each pastor is responsible for his own particular flock, not all believers in some universal, invisible, mystical flock. Flocks are separate gatherings of believers that are led by their individual pastors.
In every context of the word "flock," the metaphor speaks of an individual church, a gathering of believers. The concept of the "flock" does not back up the concept of a universal, unassembled entity at all. A flock is a church.
Taking that understanding of flock and then looking at a parallel passage, John 10, will help us to understand what Jesus talks about there. In John 10:27-29, when Jesus mentions "His sheep," who hear "His voice and follow Him," we would assume that His sheep would be a part of His flock. The term "flock" is not used in John 10, but from the other usages of "flock" in the New Testament, we would know that the way His sheep follow Him is through one of His flocks. Those outside of a church are not following Christ. They would not be His sheep. This is not to say that we are saved through church membership. It is to say that saved people will identify with Christ through baptism and by joining a "flock." There may be those who claim to be His sheep, but 1 John 2:19 says that we should not consider those not with an assembly to be "of" that assembly, that is, they would not be regarded as saved people. There is no category of unchurched saint or unflocked sheep in the New Testament.
Jesus' sheep would have a shepherd as seen in Acts 20:28-29 and 1 Peter 5:2-3. The church is the pillar and ground of the truth (1 Tim 3:15), so it is the place of Jesus' voice. Those following Christ would be hearing His voice in the flock that He started, which is local and visible.
We can understand the unity of the flock from John 10. It comes from the voice of the Shepherd, that is, unity comes from the all the Words of the Lord Jesus Christ. We live by all the Words that proceed from the Lord's mouth (Mt 4:4). His followers as a whole, even as seen in the plural pronoun "ye" in Matthew 28:18-20, observe all the things that Jesus said. Just like Jesus came to keep all the things that the Father told Him, so do His followers. We are sanctified like Jesus was (John 17:17-19), which is by all that was commanded. Jesus said the same in John 14, when He said that those who love Him are those who keep everything that He said. The unifying factor of a church is the teaching and then practice of that church. This teaching and practice is also what separates that church from the world and from other entities or organizations with a different belief and conduct.
Those who attempt to read into the concept of "flock" a universal entity are doing just that—reading into Scripture. Instead of taking the plain meaning, they start with an ecclesiology concocted by state churchism and then try to find it in the Bible. To do so, they must pervert the doctrine of "the flock," one based upon its actual usage in God's Word. You will not find a universal church in the usage of flock in the New Testament. In one sense, if there were a universal flock, it is perpetually scattered, not at all within the Biblical understanding of a church.
In John 10:16 we read a related word, "fold." In the context, the fold is Israel. Jesus will lead His sheep out of that fold. Of course, His sheep will join His flock, but what is His flock? His flock is local and visible. That flock is made up of His sheep for sure. The New Testament teaches a regenerate membership. Churches are made up of sheep. We try to keep the wolves out (Acts 20:28-29) even though that takes the skill and labor of the pastor to do so. Gentiles would come into that flock, His church. Each of His churches is His church. Gentiles would be included in the same churches as Jews, therefore, would too be a part of His fold. That whole idea doesn't change the understanding of His flock and how it is used in the New Testament. It fits into the concept that we see also in Ephesians 2 when Paul says that the barrier had been erased between Jews and Gentiles in the church. Both would follow the Lord through His flock.
Unity is found in the flock of Christ. That flock is local and visible. It has a shepherd appointed by the Holy Spirit (Acts 20:28). The flock is unified by the Word of Christ.