It would be interesting to ask what common denominator, if any, existed in the early church. Most would say that it was the proclamation that Jesus had risen from dead, yet there were some groups (such as the proto-gnostic group responsible for the Gospel of Thomas, likely located in eastern Syria) which had far more concern for the words of Jesus, and likely did not view Jesus as dying at all (that is, in the gnostic and docetic sense of the end of bodily life as a departure).
There were factions and villification between factions, but at least some shared the concept of a single, shared ekklesia, such as attested in Paul, in Matthew, and in other sources. It is interesting how Paul called on this ideology to try to bring the different Corinthian factions of Paul, Cephas, and Apollos into unity lest the body of Christ be split. There is also the interesting Matthean concept of the Devil planting lawless Christians in the church, existing alongside those who truly will receive the kingdom, and they will remain inside the group until the Son of Man and his angels remove them from his kingdom.