To answer that question, we have to back up to the beginning. In 33 AD, Jesus ascended into heaven, leaving behind a group of disciples and other followers to whom He gave instructions to wait for their Helper. The Helper showed up on Pentecost, baptizing the disciples and other followers in the Holy Spirit and kick-starting the Christian Church (Acts 2-4). From there, the apostles started preaching and teaching the Way of Christ. This continued as the Church continued to expand, spurred on by the persecution leveled at the early Christians by the Romans and the Jews.
Since Rome was an empire of cities, the basic structure of the early churches was to have a community in each city, with one bishop overseeing the city and presbyters (priests) and deacons assisting him. This structure already clearly formed by the time of Ignatius of Antioch’s writings around 107 AD (The Orthodox Church, Timothy Ware). Ignatius writes to several churches on his way to martyrdom in Rome, emphasizing that the church MUST stay close to their bishop, that he God had appointed him, and that there could be no church without the bishop (The Apostolic Fathers, Letters of Ignatius). Without the bishop, who had been appointed by an apostle (then later appointed by an appointee of an apostle and so on), there was no way for a Christian to know if they were “in the Church” or in heresy. The leadership of the bishop was paramount and at this time, usually a short time, since they were constantly being martyred.