The Bible tells us that God, "Put all things in subjection under his (Christ's) feet, and gave him to be head over all things to the church, which is his body, the fulness of him that filleth all in all" (Eph. 1:22-23). Then, three chapters later, the same Bible states, "There is one body, and one Spirit, even as also ye were called in one hope of your calling; one Lord, one faith, one baptism, one God and the Father of all, who is over all, and through all, and in all" (Eph. 4:4-6).It has become popular and ("politically correct) today to say "Join the church of your choice." But look at those passages above. In the same way that there is "one Lord" and "one faith" and "one God," there is also 'one body" and that body is "the church." The Bible never speaks of "different churches." In God's sight, there is only one church, and that is the one which Jesus built. As He said, "I will build my church" (Matt. 16:18). He never built but one, the one that is the "one body" of Eph. 4:4. In fact, Jesus prayed that all believers would be "one" in Him (John 17:20-21).
Jesus died for the church (Eph. 5:25-27), and the Bible says, "For the husband is the head of the wife, and Christ also is the head of the church, being himself the savior of the body" (Eph. 5.23). Please note that Christ died for only one church, He purchased only one church with His blood and He is the Savior of only one church. That is what the Bible says.
When people on Pentecost (Acts 2) believed in Jesus to be "both Lord and Christ" (vs.36), they asked what to do, and the Lord (by the mouth of Peter) told them, "Repent ye, and be baptized every one of you in the name of Jesus Christ unto the remission of your sins (vs. 38). We then read: "They then that received his word were baptized: and there were added unto them in that day about three thousand souls...And the Lord added to the church day by day those that were being saved (Acts 2:41,47).
When the Lord saves people (Acts 2:38), He adds them to that one church (Acts 2:41,47). All of those who are saved are in that one church because the Lord who saves them put them into that one church.
The one church would be the church of Christ because HE purposed it (Eph.3:10-11), HE built it (Matt. 16:18), HE purchased it with His own blood (Acts 20:28), HE adds the saved to it (Acts 2:47),and HE tell us, "All the churches of Christ salute you" (Romans 16:16).