From the day of Pentecost onward the Good News had been spreading out in all directions from Jerusalem. However, during this era of slow travel and equally slow communication there was infrequent uniformity of thought and practice between local fellowships of Jesus’ followers. Somebody had to give authoritative instructions to the new Christian movement or it could fracture into disparate faiths. Therefore, sometime between twelve and twenty years after Pentecost, the first universal letter to be written and sent to all new communities of Jesus’ followers.
The Acts of the Apostles, Part 33 – Outline (PDF)