We see this explicitly reported in Paul's letter:
Colossians 4:16 -- When this letter is read among you, have it also read in the church of the Laodiceans; and you, for your part, read my letter that is coming from Laodicea.1
It is also true of documents post-dating the New Testament:
This helps show that the early churches were well-networked because of the amount of intimacy it conveys between the churches; Paul's request was not awkward but likely commonplace and natural.
Richard Bauckham: “Since we have noticed that the letter of James seems to be addressed to the eastern as well as the western Diaspora, it is important also to notice evidence of Jerusalem’s connection with the Christian movement there.” [James: New Testament Readings (Routledge, 1999), 18.]
Kurt & Barbara Aland: “The earliest writings to be collected were probably the letters of Paul. Each of the churches having one or more letters from the apostle would not only preserve them carefully, reading them when they assembled for worship, but would also exchange copies of their letters with neighboring churches. This is the only possible explanation for the preservation of the Galatian letter, since the church(es) addressed in it did not survive for long. … Whether written by Paul or written shortly after his death, this [asking churches to share letters] reflects in all probability the practice of the Pauline (or post-Pauline) period.” [The Text of the New Testament (1995), 48.]
The early Christian churches throughout the Mediterranean mirrored each other in terms of how they worshipped and practiced the Christian life. For example, Christians universally worshipped on Sunday, which even replaced Saturday as the holy day for Jewish-Christians. This is no small feat, and suggests intimate coordination between the brotherhood.
We see an abundance of evidence that Christians often travelled in-between the various communities without a disruption in shared beliefs.
Early Christians were thoroughly networked and quite cooperative in the 2nd century.1 This is relevant to whether early Christians (in AD 30-7) were well-networked because the reasons for their unity apply just as well if not more so to them:
- Harry Gamble: “As I have shown, the circulation of Christian literature was private, being part and parcel of the constant intercourse between individual congregations. Transmission took place by letter and messenger (letters requiring couriers) throughout the first five centuries of the church. It is no less typical of Augustine in the fifth century than it was of Cyprian in the third century, of Polycarp in the second, or of Paul in the first, to mention only a few examples. The travel of individual Christians or small delegations from one church to another, often over large distances, made the variety and breadth of Christian literature known to the congregations, thus increasing interest and demand, and also served as the efficient vehicle for the brisk movement of texts from one place to another.” [Books and Readers in the Early Church (Yale, 1995), 142.]
There are several reasons for why early Christians would want to be well-networked.
We will get into reasons such as this:
John 13:35 -- By this all people will know that you are My disciples: if you have love for one another.”
Philippians 1:27-28 -- Only conduct yourselves in a manner worthy of the gospel of Christ, so that whether I come and see you or remain absent, I will hear about you that you are standing firm in one spirit, with one mind striving together for the faith of the gospel; 28 and in no way alarmed by your opponents—which is a sign of destruction for them, but of salvation for you, and this too, from God.
Christian churches in AD 30-70 basically unified, e.g. in their leaders, doctrine, & history.