If Scripture is the only infallible rule of faith and practice for the church, then it is vital that we know which books constitute Scripture. There are, after all, many books that claim to be from God or that others claim are from the Lord. How, then, do we identify what the Lord has inspired and what He has not?
Discerning the Old Testament canon is relatively easy, as we have seen. If Jesus is Lord, then we want to have the canon that He followed, and we know that His Old Testament canon was the thirty-nine-book Protestant Old Testament canon.
Things are more complicated when it comes to the New Testament. Yet, church history shows that there was an early consensus about the New Testament canon. Certain books—including the four Gospels, the Pauline Epistles, the Acts of the Apostles, 1 Peter, and 1 John—were universally accepted, with almost no one doubting their Scriptural status. Some early believers, however, had questions about books such as Revelation and 2–3 John. In the end, certain objective factors helped move the church to receive these books as Scripture: they had a credible claim to Apostolic authorship, taught in accord with the other unquestioned books, and were read in churches in all parts of the known world. By the middle of the fourth century A.D., the church had settled on the twenty-seven books of the New Testament, and the Protestant Reformers affirmed this canon just as the Roman Catholics did.
Although the aforementioned objective factors regarding the New Testament books were appealed to as the church was discerning the scope of Scripture, the reception of the canon also involved subjective factors as well. Because Scripture is from God Himself and because there is no authority higher than the Lord, the final reason why the church received the canon it did was due to its hearing the voice of God in the pages of the received books. While objective evidences for canonicity are persuasive and necessary, we are finally convinced to receive Scripture as Scripture by the work of the Holy Spirit in the hearts of His people. John Calvin wrote: “These words [of Scripture] will not obtain full credit in the hearts of men, until they are sealed by the inward testimony of the Spirit. The same Spirit, therefore, who spoke by the mouth of the prophets, must penetrate our hearts, in order to convince us that they faithfully delivered the message with which they were divinely entrusted” (Institutes 1.7.4).