structural:summarization, information:classification
The phrase New Testament as the collection of scriptures

The use of the phrase New Testament (Koine Greek: Ἡ Καινὴ Διαθήκη, Hē Kainḕ Diathḗkē) to describe a collection of first- and second-century Christian Greek scriptures can be traced back to Tertullian in his work Against Praxeas. Irenaeus uses the phrase New Testament several times, but does not use it in reference to any written text. In Against Marcion, written c. 208 AD, Tertullian writes of:

the Divine Word, who is doubly edged with the two testaments of the law and the gospel.

And Tertullian continues later in the book, writing:

it is certain that the whole aim at which he has strenuously laboured, even in the drawing up of his Antitheses, centres in this, that he may establish a diversity between the Old and the New Testaments, so that his own Christ may be separate from the Creator, as belonging to this rival God, and as alien from the law and the prophets.

By the 4th century, the existence—even if not the exact contents—of both an Old and New Testament had been established. Lactantius, a 3rd–4th century Christian author wrote in his early-4th-century Latin Institutiones Divinae (Divine Institutes):

But all scripture is divided into two Testaments. That which preceded the advent and passion of Christ—that is, the law and the prophets —is called the Old; but those things which were written after His resurrection are named the New Testament. The Jews make use of the Old, we of the New: but yet they are not discordant, for the New is the fulfilling of the Old, and in both there is the same testator, even Christ, who, having suffered death for us, made us heirs of His everlasting kingdom, the people of the Jews being deprived and disinherited. As the prophet Jeremiah testifies when he speaks such things: "Behold, the days come, saith the Lord, that I will make a new testament to the house of Israel and the house of Judah, not according to the testament which I made to their fathers, in the day that I took them by the hand to bring them out of the land of Egypt; for they continued not in my testament, and I disregarded them, saith the Lord."... For that which He said above, that He would make a new testament to the house of Judah, shows that the old testament which was given by Moses was not perfect; but that which was to be given by Christ would be complete.

Eusebius describes the collection of Christian writings as "covenanted" (ἐνδιαθήκη) books in Hist. Eccl. 3.3.1–7; 3.25.3; 5.8.1; 6.25.1.