Let's begin by asking: what is this historic faith? Jude 1:3 says this: "Beloved, although I was very eager to write to you about our common salvation, I found it necessary to write, appealing to you, to contend for the faith that was once for all delivered to the saints." That phrase, "the faith once delivered," describes a faith that was given, not invented; complete, not evolving; divine, not humanly modified.
We can call it the historic Christian faith, the ancient faith, the apostolic faith, or simply the faith that was once for all delivered to us. What they handed down was complete, authoritative, and final, not an unfinished draft waiting for modern improvement. The faith was given once for all, and our task is not to rewrite it, but to return.