Meaning, the rule of faith that encompasses the Sacred Scriptures. I was thinking the other day about how well Piepkorn nailed this:
"The term "Canonical" in Christian tradition is always relative; it refers to the actual canon in use in a given diocese or province at a given time. The content of the canons varies from time to time and from place to place. The canon was never fixed for the whole Church by an ecumenical Council." (The Sacred Scriptures and the Lutheran Confessions, p. 4)
At times, you will hear criticism of Lutherans appealing to the Fathers as ignoring the context in which those Fathers operated, including their "canon." But that is a particularly interesting point.
The canon that the Cappadocian fathers operated with (assuming, as I am, that St. Gregory of Nazianzus represented the same approach as Sts. Basil and Gregory of Nyssa) was closer to the typical "Protestant" canon of today than to the typical Roman or Orthodox canon. He enumerated basically the same books that canon, leaving out Esther and Revelation. Regarding anything other than those books, he says: "If there's anything else besides these, it is not among the genuine." (On God and Man, p. 86)
A few years before the time of their ministry, a canon is provided for us by St. Athanasius the Great in his festal letter #39. He lists basically the OT canon of Protestants today, with the exception of including Baruch among the genuine books. For the NT he lists the common canon. Of these books he says: "These are fountains of salvation, that they who thirst may be satisfied with the living words they contain. In these alone is proclaimed the doctrine of godliness. Let no man add to these, neither let him take ought from these. For concerning these the Lord put to shame the Sadducees, and said, 'You err, not knowing the Scriptures.' And He reproved the Jews, saying, 'Search the Scriptures, for these are they that testify of Me Matthew 22:29; John 5:39.'"
He goes on to speak of more books, though, which he describes as follows: "there are other books besides these not indeed included in the Canon, but appointed by the Fathers to be read by those who newly join us, and who wish for instruction in the word of godliness. The Wisdom of Solomon, and the Wisdom of Sirach, and Esther, and Judith, and Tobit, and that which is called the Teaching of the Apostles, and the Shepherd. But the former, my brethren, are included in the Canon, the latter being [merely] read"
Does that sound familiar? Luther introduces the Apocryphal books with these words: "Apocrphya: that is, books, which though we do not hold to be like Sacred Scripture, yet are useful and good to read."
Meanwhile, in the West, Canon 24 of the Council of Carthage (with which St. Augustine's writings also concur), lists the broader canon including the so-called Deuterocanonical books as canonical books to be read in the Churches and forbidding the reading of non-canonical works in the Churches.
As you can see, I think Dr. Piepkorn's point is amply substantiated. Speaking as a Lutheran Christian, one of the sadder things to happen in the transition to the English language in the Missouri Synod was the loss of the Apocrypha from our Bibles. Every Bible CPH printed in German had them (with Luther's introductory words cited above), I believe, and in the lectionary listing in the back, even appointed them for reading on certain festivals. A pity that at that time the form of the AV that had become popular in the US didn't any longer have the full set of books, as the original AV had had.
I keep hoping that there will be a version of the ESV that will provide us with a decent translation of the Apocrypha. Someday, maybe?