Ignatius speaks of the role of the Bishop in the unity of the church.
"For your justly renowned presbytery, worthy of God, is fitted as exactly to the bishop as the strings are to the harp. Therefore in your concord and harmonious love, Jesus Christ is sung. And do ye, man by man, become a choir, that being harmonious in love, and taking up the song of God in unison, ye may with one voice sing to the Father through Jesus Christ, so that He may both hear you, and perceive by your works that ye are indeed the members of His Son. It is profitable, therefore, that you should live in an unblameable unity, that thus ye may always enjoy communion with God. (Ignatius to the Ephesians, Chapter 4)In the early church, the bishop and presbyters provided oversight to the church, much like the elders in ancient Israel. However, the bishop represented the extension of the apostles and, as such, were the final arbiter of truth. They were the final authority as to what Jesus taught his apostles and what the apostles taught and what they meant by what they wrote. They were the "keepers" of the apostolic message of which the church was the "pillar and support of the truth." (1 Timothy 3:15) They were the living definition of what it meant to be orthodox. This is why to be outside the fellowship of the bishop was to be outside the church.
"Let no man deceive himself: if any one be not within the altar, he is deprived of the bread of God. For if the prayer of one or two possesses such power, how much more that of the bishop and the whole Church! He, therefore, that does not assemble with the Church, has even by this manifested his pride, and condemned himself." (Ignatius to the Ephesians, Chapter 5)It is so hard to place ourselves back onto the situation that the early church experienced since we have drifted so far from the early church model. However, in their day they did have those who set themselves up against the bishops and the apostolic churches. Men such as Valentinian and Simon had setup their own churches with their own bishops and even their own love (or agape) feasts. They used the same scriptures and the same names for God but they preached a gospel that was radically different from the one the church had received. They were men whom Polycarp would call the "Firstborn of Satan." The evidence of their error was not just their teaching but also that their bishops had no connection (or lineage) back to the apostles and their churches could not trace the heritage back to any apostolic church. They were completely found "outside" the altar.
Ignatius reminds us of one very specific advantage to unity, when he said, "thus you will always enjoy communion with God." When there is strife and discourse in the church, our communion with God suffers. However, when we exist harmoniously with each other and with the bishop and the presbytery then our hearts are united together and our lives are as a song that rises up to God. In such unity there is not only an individual communion with God but also a corporate communion; in such unity, God is present.