It has often been argued that the heresies were one of the main reasons why the canon was formed, but according to Allert this is a half-truth. For instance, Marcion's insistence on the usage of Luke + Paul - the Old Testament may have been more concerned with rejecting certain writings than with assembling a definite collection. Marcion's collection was an influence on the emerging canon, but was not determinative for it. The writings revered and cited by the Church long after Marcion remained fluid and unfixed. When it comes to the Gnostics, the problem was not so much that they had their own set of Scriptures as much as it was that they were alleged to have pervereted the interpretation of the Church's own Scriptures. The issue with the Montanists was their claim to prophetic authority and not an attack of the authority of a recognized body of literature known as the canon.
Next Allert goes on to discuss the Muratorian Fragment (which mentions positively the Shepherd of Hermas, the Wisdom of Solomon, and the Apocalypse of Peter but raises doubts about their usage in public readings). The dating of the fragment is of course disputed and arguments include the late second and early fourth century. Graham Stanton has convinced me of a second century date (esp. with the references to the Shepherd of Hermas), but regardless of the dates Allert points out that everyone agrees that (1) by the end of the second century the four Gospels, Paul's letters, 1 Peter and 1 John were also used widely and had attained authoritative status, (2) the status of other writings varied throughout the second, third, and fourth centuries, (3) lists that restrict or delimit the scope of authoritative writings derive mainly from the fourth century. Allert writes: "Thus, rather than conceive of a closed New Testament canon in the second century, or even documents on the way to becoming canonical, we must shift our concerns to how authoritative documents functioned in the church and what these documents were" (p. 108).
On the subject of the Gospels in the second century, Allert is quick to point out that acquintance with, knowledge of, and citation from the Gospels does not mean that they were necessarily recognized as canonical or that there was an explicit four-fold canon. Interestingly enough, Allert points out that the Apostolic Fathers were interested in the words, teachings, and commands of Jesus from either oral or written sources. That is why we findstatements like: "remember the words of the Lord Jesus Christ" (1 Clem. 46.7). Viewed in this way, the Apostoic Fathers really were Red Letter Christians!
Justin Martyr is well known for his references to the "Memoirs ofthe Apostles" as his way of making reference to the Gospels. Justin may have used a harmony of some kind (I'm not quite as confident as Allert of this) and Justin incorporates certain non-biblical traditions such as the Magi coming from Arabia. Allert also mentions Gospel harmonies by Tatian, Ammonius of Alexandria, and Theophilus of Antioch. Tatian's Diastessaron was probably not named so by Tatian since it is given different names by later Christian authors. The Diatessaron also includes the variant tradition fo a great shining light at Jesus' baptism (known also to Justin Martyr and Epiphanius and perhaps in Gospel of the Ebionites). The Diatessaron itself was probably the first "Gospel" to reach Syria and it functioned as Scripture in the Syrian church up until the fifth century.
On Irenaeus of Lyons, Allert notes how he provided a concerted defense of the four-fold Gospel. The question is whether Irenaeus was being innovative at this point or whether he was providinga cosmic appropriateness to a widely accepted view of four Gospels in the Church? I favour the latter but that does not make the Gospels necessarily canonical. For Irenaeus the canon was the "canon/rule of faith" which tempered the exclusivity of the four Gospels as canon. The Rule of Faith itself was not rigidly defined, but a fluid and elastic articulation of the main doctrinal affirmations of the second century church. The Rule of Faith provided the context in which Scriptured was to be decided and interpreted. Allert concludes that: "There is no doubt that the Gospels known to us as canonical were authoritative in the second century. But these Gospels were not the exclusive source for faith and doctrine in the early church" (p. 126). Consequently Scripture (esp. Gospels and Paul) functioned authoritatively in the early church, but it functioned so within a wider ecclesial traditon.