William Wrede famously said that “anyone who accepts without question the idea of the canon places himself under the authority of the bishops and theologians of those centuries.” If Protestants question the ancient catholic church’s judgements and authority, a consistent Protestant will also contest their limitations of the extent of the canon. Thus, for Wrede, NTT is really about plotting the debates, developments, and diversities in early Christians literature, including but looking well beyond the New Testament, into the age of the apostolic fathers and finding its upper limit with the early apologists. Here NTT is merely a cipher for history of Christian religion and is the attempt to ascertain “what was believed, thought, taught, hoped, required, and striven for in the earliest period of Christianity; not what certain writings say about faith, doctrine, hope, etc.” 
Should we abandon New Testament Theology for the History of Early Christianity? Well, as for equating submission to the biblical canon with submission to episcopal authority, well, all I can say is give me a fourth century Cappadocian bishop speaking ex cathedra any day over a twentieth century liberal German protestant, because we all know the wonderful fruits born by liberal German Protestantism between 1914 and 1945.
Against those who insist that one must look beyond the church’s canon, it should be remembered that the canonisation of the New Testament was born of necessity and did not so much narrow the faith as much as prevent it from becoming so broad as to be nebulous. The formation of the canon was a historical process of theological judgments, one that was inevitable as it was essential. The question raised by persecution (which books do I hide from authorities) and deviation (which books should we not read at church) had to be settled as a pragmatic necessity, whether that was locally (e.g. Bishop Serapion in Antioch in relation to the Gospel of Peter ca. 180 AD), in a large diocese (Athanasius in his 39th Festal Letter of 367 AD), or regionally (councils of Hippo and Carthage ca. 393-419 AD). The New Testament’s canonisation represents the coalescing of a creedal and textual culture, however diverse it was across the east and west, that settled on the books of the New Testament as the normative attestation of the church’s faith. The formation and promulgation of the New Testament canon was not an arbitrary arrangement nor an exercise in pernicious politics. It constituted an effort to retrieve the christological foundations and apostolic testimony on which the church was based and to set limits on the extent of the diversity which it already knew. The canon represented an attempt to apprehend authentic apostolicity, to arrest diversity headed towards deviancy, all the while operating within a consensus of broad catholicity. Its effect was to doubly to unify diverse groups around an agreed apostolic heritage and to rule out dissidents whose views lacked apostolic coherence. The limits of diversity could not accommodate splitting the God of creation from the God of redemption (Marcionism), entertaining strange cosmologies while furnishing Jesus as an esoteric revealer (Valentinianism), worshipping a human Jesus and holding a Torah-centred faith (Ebionism), or lifting up new prophecy to absolute authority (Montanism).
 Wrede 1973 , 71.
 Wrede 1973 , 84-85.
 See Schröter 2013 , 326-27, 337-39.
 Markschies 2015, 246.
 Schröter 2013 , 327.
 See Gamble 1995, 141.
 Dunn 2007, 241.
 Hence Berger (1994) whose book is titled Theological History of Early Christianity (Theologiegeschichte des Urchristentums) and subtitled Theology of the New Testament (Theologie des Neuen Testaments). Theodore Zahn (1928, 1) endeavoured to marry together a focus on the New Testaments’ presentation of a theology as it was apparent in its “historical development” and “ordered according to the steps of salvation history.”
 Schröter 2013 , 337-40, 48-49.