Evangelicals assert that wherever there is ecclesia, an assembly of believers, there is the church of Christ. Multitudes were gathered to hear Christ preach the Sermon on the Mount (Matthew 5:1). Believers followed Him to Peter's house (Matthew 8:14-16). His disciples followed Him wherever He went (Matthew 8:23). In Matthew 13, the crowds were so large that He had to get in a boat to speak to them on the shore (Matthew 13:1-2).
In Matthew 14, Jesus fed 5,000, many of whom were healed of sicknesses (Matthew 14: 13-21). How could they be healed and miracles occur if there were not believers in their midst? Obviously, there were. In Gennesaret, many people touched the hem of His garment and were healed (Matthew 14:34-36). Four thousand believers experienced healing and other miracles by the Sea of Galilee (Matthew 15:29-39).
If the church of Christ truly was constituted by a simple gathering of believers, then the church had already been established at the Sermon on the Mount, beside the lake, at Gennesaret, and by the Sea of Galilee. But let's read further. Then Christ came to the region of Caesarea Philippi:
When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? And they said, Some say that thou art John the Baptist: some, Elias; and others Jeremias, or one of the prophets. He saith unto them, But whom say ye that I am? And Simon Peter answered and said, Thou art the Christ, the son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.
Assuming that the church already existed at the mount, by the lake, at Gennesaret, and by the Sea of Galilee, why then did Jesus say in Matthew 16 "I will build my church" as if this would happen in the future?
It must be that the original evangelical premise is faulty. The church of Christ is not simply a gathering of believers. There must be other elements of the church that had yet to be established. I Corinthians 12:28 tells us that there were "helps" and "governments" to bless the New Testament church.
When He arrived at Caesarea Philippi, Christ had not yet established His church, yet He promised that He would do so in the future. This is because the church of Christ is comprised of more than simply a gathering of believers. It is an assembly of believers united by formally called and ordained ministry and holy ordinances.
After the cross, the New Testament church clearly had helps and governments that Christ had not yet revealed to His disciples in Matthew 16. In Matthew 16, Christ's church had not yet been established. When Christ said "I will build my church," He was explaining to His disciples that the other elements of His church would soon be established.