Commentaries:
By verse 5, he is already talking about the truth. He gives its source to be sure that nobody misunderstands. There is truth about many things—it could about an automobile or any other kind of product, and there can be truth about any kind of a human circumstance or event. Paul, however, is writing about a specific truth, which deals with what is contained within the gospel. The gospel is a certain message of good news. Thus, Paul begins to establish the source of this specific truth: It came from whomever brought the good news to them, the one whom God used to establish their congregation.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty)