According to Warfield, the church did not create a new canon alongside the old by determining what ought to be included in it (or not). Rather, the church recognized the books of our present New Testament as they were given, and therefore added them to the existing books of the Old Testament canon. These books came from the apostolic circle under the inspiration of the Holy Spirit and therefore already possessed full authority as the word of God independently of the church’s recognition of them.
B. B. Warfield’s magisterial essay “The Formation of the Canon of the New Testament” was published in 1892. You can find the essay here. It has also been included in the various editions of Warfield’s The Inspiration and Authority of the Bible.
Here a few gems from that essay.
Warfield reminds us that the apostolic church did not “invent” the idea of a canon of New Testament books. The church possessed a canon of inspired and authoritative books from the very beginning–the Old Testament. The church was, therefore, never without a “canon.”
In order to obtain a correct understanding of what is called the formation of the Canon of the New Testament, it is necessary to begin by fixing very firmly in our minds one fact which is obvious enough when attention is once called to it. That is, that the Christian church did not require to form for itself the idea of a “canon” — or, as we should more commonly call it, of a “Bible” — that is, of a collection of books given of God to be the authoritative rule of faith and practice. It inherited this idea from the Jewish church, along with the thing itself, the Jewish Scriptures, or the “Canon of the Old Testament.” The church did not grow up by natural law: it was founded. And the authoritative teachers sent forth by Christ to found His church, carried with them, as their most precious possession, a body of divine Scriptures, which they imposed on the church that they founded as its code of law. No reader of the New Testament can need proof of this; on every page of that book is spread the evidence that from the very beginning the Old Testament was as cordially recognized as law by the Christian as by the Jew. The Christian church thus was never without a “Bible” or a “canon.”
Through the revelation of the gospel preached by the apostles, Warfield notes that the Holy Spirit added to the existing Old Testament canon during the apostolic age. During this period of its development, the church did not possess a “closed canon,” but “an increasing canon.”