This is a subsidiary post of The Study of Canon and the Etymology of Certain Terms.
Biblically speaking, the terms “new testament” and “new covenant” are identical. Either way, the term refers to the new relationship of God and His people prophesied by Jeremiah 31:31-34 and fulfilled in Jesus Christ as described by Hebrews 8:8-12. However, subsequent to biblical times the term “New Testament” has also come to refer to the collection of writings produced by the apostles of Jesus Christ as distinguished from those produced before them by the prophets of God, which, of course, are themselves collectively called the “Old Testament.”
As for using the term “old testament” or “old covenant” as a reference for a set of writings, Paul uses the term this way in 2 Corinthians 3:14:
But their minds were hardened; for until this very day at the reading of the old covenant the same veil remains unlifted, because it is removed in Christ.
Since neither the Old Testament or the New Testament uses the term “New Testament” to refer to a collection of writings, the question arises: How and when did this use of the term come to be? For today, whenever anyone hears the term “New Testament,” the collection of apostolic writings comes immediately to mind.
More information on the sources for the quotes below can be found by refering to Annotated Bibliography on New Testament Text and Canon.
Kostenberger, Andreas and L. Scott Kellum and Charles L. Quarles. The Cradle, the Cross, and the Crown: An Introduction to the New Testament. B&H Academic, 2009. Kindle location 852.
Finally, the title of the whole collection is “The New Covenant.” This is the term for the collection used by Irenaeus (c. 130– c. 200); Clement of Alexandria (c. 150– c. 215); Tertullian (c. 160-c. 225); and Origen (c. 185-c. 254). Perhaps the most convincing statement is by Apolinarius’s unknown associate (2d cent.): “I was somewhat reluctant , not from any lack of ability to refute the lie and testify to the truth, but from timidity and scruples lest I might seem to some to be adding to the writings or injunctions of the word of the new covenant of the gospel, to which no one who has chosen to live according to the gospel itself can add and from which he cannot take away.” From this quote it is apparent that this presbyter in the second century had a closed canon that he called “the writings of the new covenant of the gospel.”
Litfin, Bryan M. Getting to Know the Church Fathers: An Evangelical Introduction. Brazos, 2007, 304 pages.
The earliest believers did not have any awareness of a second “Testament” to complement the first. For them the scriptures were the sacred writings that Jesus himself had read and interpreted. It took some time for the church to realize that another distinct Testament had been delivered to the human race. (Kindle location 1616)
It may be surprising to learn that Irenaeus was the first person to use the tem “New Testament” to mean what we mean today. Before his time, there was no awareness within the church that the sacred books of Christianity comprised a second body of writings alongside the received Jewish scriptures. The earlier fathers certainly possessed books they revered as holy , but did not yet think in terms of a well-defined second testament. The process of determining which books were actually in the canon of scripture was just starting to occur around the time Irenaeus was pastoring his church in Lyons. (Kindle location 1333)
As we said in chapter 3, Irenaeus was the first to use the term “New Testament” in connection with a body of writings. A little later Tertullian was even more clear in his usage of this term. In fact, he is the church father to whom we assign the honor of giving us the literal term “New Testament” (from the Latin novum testamentum). If Irenaeus was somewhat vague about what he meant, two decades later Tertullian certainly operated with a conception of the “New Testament” as a distinct scriptural corpus. We see, then, that at the dawn of the third century a new awareness arose among the Christian faithful: that their Bible was comprised of both an Old and a New Testament. The church continued to function with this awareness for more than a century, until Emperor Constantine’s imperial acceptance of Christianity finally enabled church leaders to make some official pronouncements about the extent of the biblical canon. (Kindle location 1637)
Trobisch, David J. “Canon: III. Formation of the New Testament” in Encyclopedia of the Bible and its Reception Vol. 4, ed. by Hans-Josef Klauck et al. De Gruyter, 2012, p. 897-901. (linked from Trobisch’s website)
Melito of Sardes (ca. 180 CE) is usually cited as the oldest witness to the title “New Testament.” Although he does not use the term explicitly, it is implied by his use of the title “Old Testament,” which is introduced without further explanation (Eusebius, Hist. eccl. 4.26.13–14). At the end of the 2nd and the beginning of the 3rd century CE Irenaeus, Clement of Alexandria, Tertullian, and Origen refer to the NT as a collection, indicating its wide circulation (Trobisch 2000, p. 44, bibliography).