So when did the Lord's church, "My Church" (Matt. 16:18), begin? A study of Scriptures should make this event clear. The first church was organized out of the material prepared by John the Baptist (Acts 1:22). John received his authority to baptize from heaven (Mt. 21:25). Jesus and all the apostles including Matthias were baptized by John (Acts 1:21-22). Jesus took this material prepared by John the Baptist and organized His church.
"My Church" was in existence during the earthly ministry of Christ. Jesus said in Matt. 16:18, "I Will build my church," meaning during the days of His ministry on earth He would begin building it. In Matt. 18, the authority of discipline was placed in the church during Christ's earthly ministry. According to Heb. 2:12 and Mark 14:26, Christ sang God's praise in the midst of the church. According to Acts 1:15-26 and John 13:29, the church had a treasurer and organization during Christ's earthly ministry. Christ also gave the Great commission to the church in Matthew 28:16-20.
The church, therefore, was established in the days of Jesus' ministry here on this earth. It is in the Gospels, during the ministry of Christ that we see a true church by definition: "a local visible body of organized baptized believers. Many look to Pentecost as being the birth of His church; but on that day 3,000 were 'added' unto them. There has to be something already in existence in order to be added unto, thus the church was already in existence.
Thus the church began during the ministry of Christ while on this earth. That was the birth of His church. It was that church that was given instruction for discipline (Matt. 18), command to keep the ordinances (Matt. 26 & 28), and the Great Commission (Matt. 28:19-20). It was that church through preaching and the work of evangelists, that reproduced itself and literally turned the world upside down (Acts 17:6). Our Lord gave to His church the ordinances, discipline, doctrine, the commission, His power, and His authority; and according to Ephesians, there is truly only one Lord, one faith, and one baptism (vs. 5).
So if a church traces its beginning to a man and not Christ, does not keep the ordinances, practice discipline, continue in doctrine nor fulfill the commission; it's only pretending.