‘Now in these days there came down prophets from Jerusalem to Antioch.'
It began when prophets from Jerusalem came down to Antioch, presumably because they had heard of the work that was going on and wanted to assist. It was a further expression of the love and concern of the church in Jerusalem for this new church made up of a combination of Jews and Greeks. We know of these prophets from 1 Corinthians 12-14. Their Spirit-inspired expounding of the word could, if wisely used, be a great encouragement and strength to new believers. (Compare 1 Corinthians 14:3; 1 Corinthians 14:31; Ephesians 3:4-5). And occasionally, but not often, such prophets would receive ‘a revelation' concerning the future (at which point all other prophets had to give way. A ‘revelation' was the only grounds for interrupting a prophet who was prophesying - 1 Corinthians 14:30).
Such prophets, if they taught wisely would be a great help. They were well founded in the Old Testament and the Testimony of Jesus (the recognised tradition about Jesus' life and teaching), and were inspired by the Spirit in their presentation of them. The local prophets (inspired preachers), being still new to Christianity, would not have the same depth of knowledge of the Scriptures.