By the end of the second century, Christians and Christian writings had spread to every corner of the Roman Empire. And with this increase came an increase in quotations, allusions, and citations of New Testament writings. The research of scholars such as Franz Stuhlhofer, Biblia Patristica, and Bruce Metzger is invaluable in understanding the quality and quantity of these uses by the early Church Fathers. From a detailed study of second century writings, we notice a distinct pattern of use. Excluding Old Testament allusions and quotations, the use of specifically Christian writings (that is, those written after the death and resurrection by those claiming to follow Jesus of Nazareth) fell into three categories: books viewed as authoritative and scriptural, books viewed as non-scriptural, and “fringe” books. Among the books generally accepted as scriptural (and thereby part of the “practical canon”), a core of immediately emerged as the chief texts of Christianity, including the Synoptic Gospels (especially Matthew), the Fourth Gospel (John), and the major Pauline epistles. Metzger and Barton indicate that surrounding these core Christian texts were two additional categories of “practically canonical” writings: writings which were generally accepted but less used than the core texts, and writings which were candidates for the fringe category. By the second and third centuries those categories had become clearly differentiated, with books such as the Acts of the Apostles, the minor Pauline letters, the Catholic Epistles, and the Apocalypse of St. John, being widely viewed as scriptural and texts such as First Clement, the Epistle of Barnabas, the Shepherd of Herman, and the Didache generally categorized as “fringe” writings.
For many scholars the two key figures in the transformation of the writings from somewhat authoritative to canonical scripture are Justin Martyr and Irenaeus of Lyons. In Justin’s writings (ca. 150-160 CE) we first see the texts of the New Testament referenced as sacred scripture— using the “it is written” citation in reference to the Gospels. Irenaeus (writing ca. 170-180 CE) was the first Father to explicitly promote a four gospel canon; he further developed the place of the New Testament writings by being one of the first to refer to the texts of the New Testament as “scripture.” Thus, by the mid- to late-second century, we see clear examples of the canonical Gospels referenced as authoritative scripture and forming a “mini” formal canon. Many scholars see these explicit references to the gospels not as a sign of their being advocated as scriptural, but rather as attempts to further solidify their sacredness and authority. For example, Irenaeus does not argue that the four gospels are scripture so much as he claims there should only be four gospels—the closing of the gospel canon, as it were. It was this argument for ‘closing’ the canon of the gospels that Campenhausen refers to when we writes that “Irenaeus, so far as we can tell, was the first catholic theologian who dared to adopt the Marcionite principle of a new ‘scripture’ in order to use it in his turn against Marcion and all the heretics.” 
 John Barton, Holy Writings, Sacred Text: The Canon in Early Christianity, Westminster John Knox Press, Louisville, 1997, 16-24.  Bruce M. Metzger, The New Testament: Its Background, Growth, and Context, Abingdon Press, New York & Nashville, 1965, 274-275.  Barton, 17.  Metzger, 273-275; Barton, 16-21.  Barton, 18-19.  Metzger, 275.  Lee Martin McDonald, The Biblical Canon: Its Origins, Transmission, and Authority, Hendrickson Publishers, Peabody, MA, 2007, 285-289.  Ibid., 289-301.  Ibid., 294-297.  Hans Von Campenhausen, The Formation of the Christian Bible, Fortress Press, Philadelphia, 1972, 186