Why did Paul insist that he had told them everything?

An interesting point came up in the sermon on Acts 19-20 today: Why did Paul go overboard in insisting that he had told the Ephesian elders the whole counsel of God and not held back anything at all?  Because “false teaching” invariably begins by implying that the apostolic gospel is insufficient: “If you want to have REALLY full Christian life…”

Interesting thought…