Old and New Testaments
We divide Scripture into its two Testaments, the Old and the New. Of the Old Testament, which has the covenant of grace under a testator who was yet to die, we, together with the ancient church of the Jews, to whom the "oracles of God" during this period are said to have been entrusted (Rom. 3:2), receive only thirty-nine books. These the Jews reference as twenty-two books, according to the number of the letters of their alphabet, and divide into three classes, namely, the Law, the Prophets-both the former and the latter-and the Writings. In Luke 24:44, the Savior seems to allude to this distribution. Christians separate them into the Law and the Prophets (Acts 28:23), into Moses and the Prophets (Luke 16:23, 29, 31; 24:27), and they distribute the prophetic books again into historical, dogmatic, and prophetic books. Of the New Testament, which has the covenant of grace under a testator who has already died, we number twenty-seven books, arranged into historical and dogmatic books, as well as one prophetic book.
Petrus van Mastricht. Theoretical-Practical Theology: Prolegomena, ed. Joel R. Beeke, trans. Todd Rester, vol. 1 (Grand Rapids, MI: Reformation Heritage Books, 2019), 120-121