One of the problems with coming up with a New Testament Theology is the many diversities within the New Testament, this poses a genuine problem for interpreters who are trying to identify a theological core in the New Testament, but the problem is not insurmountable.
To restate the problem, the differences across the New Testament collection are well rehearsed: Paul versus the Jerusalem church, the Synoptics versus John, the Lucan Paul and versus the Paul of the epistles, John the Elder vs the docetists and secessionists, and then in the second century the proto-orthodox church versus the Valentinians, Marcionites, and Ebionites. Concrete examples of this diversity and divergence in the New Testament are not hard to find. Although I am convinced that one can easily reconcile James’ and Paul’s account of “faith” and “works,” nonetheless, one will have to admit that they differ markedly over the application of Gen 15:6 to Christ-believers. Whereas James incorporates a standard Jewish interpretive strategy of reading Gen 15:6 in light of Gen 22:9-18 (see 1 Macc 2:52), this is an approach that Paul explicitly rejects in Romans 4 and Galatians 3 whereby Paul disallows any attempt to read Abraham’s subsequent act of obedience back into Gen 15:6 or otherwise it would make salvation based on works and not grace. Similar disparities emerge if we look at different views on food sacrificed to idols in the New Testament. The Jerusalem council (Acts 15:28-29) and words of the risen Jesus according to John the Seer (Rev 2:14, 20) expressly forbid eating food sacrificed to idols, whereas Paul treats it purely as a matter of personal conscience (1 Cor 8:1-13; 10:25-33; Rom 14:15-23). This arguably signifies different strategies for negotiating the contamination of idolatry even if everyone agreed that idolatry should be avoided. It is diversities such as these – as well as genre, perspective, situation, intertextuality, and rhetoric – that also pose a serious challenge to identifying a theology nucleus or a type of theological centre to the New Testament. How does one find a theological centre in diverse and sometimes disparate materials? Afterall, what is prominent in Paul might be peripheral to Peter. What is affirmed in Paul and Luke-Acts (Christians can live peaceably under the aegis of the Roman empire) might be repudiated in the Apocalypse of John (Christians long for the empire to be burned to the ground). What is judged to be the central message of the New Testament might not even appear in the theology of the epistle of Jude. For many scholars, a New Testament theology is merely the chronicle of a cacophony of irreconcilable conflicting interpretations and plays for power. To be honest, I think it is clear that trying to curate the New Testament into a tidy and timeless theology with a homogenous core is deeply problematic because of the New Testament’s inherent diversities, the on-going and often unresolved debates taking place, and even the developments of views held by New Testament authors.
Taking those challenges in our stride will not, however, prevent us from detecting wider patterns of consensus, convergences, and commonalities across the New Testament. It should be remembered that “diversity” does not necessarily entail vicious rivalry, radical divergence, or outright hostility. Some divergences can be complimentary, enriching, and advantageous. I proffer that if Luke, Paul, and John sat down at a New Testament conference it would not exactly be like gathering characters from the Marvel, Star Wars, and Harry Potter universes and discovering that they have nothing really in common apart from loving friends and fighting villains. A cursory reading of Luke, Paul, and John – the three largest corpora to the New Testament – suggests that they possess far more in common than any differences of perspective, emphasis, and nuance. They would have more propensity for affiliation and affection among themselves than with deviant labelling each other over their differences.
It is observable that there are considerable commonalities and continuities among the diverse Christian groups of the first century. Put theologically there are several patterns of unity across the New Testament. Gerd Theissen detects several “religious axioms” that were shared by mainstream Christians. He asserts: “[T]he consensus of primitive Christianity is governed by two basic axioms, monotheism and belief in the redeemer. In addition, there are eleven basic motifs: the motifs of creation, wisdom and miracle, of renewal, representation and indwelling; of faith, agape and a change of position; and finally the motif of judgment.” We see this in Paul’s basic agreement with the Jerusalem leaders about the gospel which is assumed across his letters (1 Cor 15:11; Gal 1:6-9, 2:1-15) and he also assumed that churches he did not establish shared in the same “tradition” as he did (Rom 6:17). The four Gospels, for all their variety, share a pool of Jesus traditions and compose striking kerygmatic biographies with Jesus at the centre. There was already developing in the mid-first century a notion of faith as “the faith,” a distinct body of belief even if it lacked the specificity and formality of later orthodox creeds. Put materially, unity was expressed in a shared literary heritage in the Jewish Scriptures, achieved through the sharing of literature and interpretive strategies, seen in the emergence of a distinctive Christian book culture (e.g. use of codices, nomina sacra, reading aids like ekthesis), common symbols (e.g., chi-rho and icthys), seen in art found in catacombs, and reinforced by the migration of creedal and hymnic materials around Christian groups. Put phenomenally, these unities need not be limited to common beliefs and texts, but can involve certain theological intangibles such as a shared religious experience of the risen Lord and the Spirit’s effervescent life, the communal life of the church united around the eucharist, demonstrations of hospitality to believers from other regions, co-belligerence against sectarian rivals and apostates, the adoption of mutually recognized modes of worship, or a shared commitment to embody God’s love in Jesus’ name. Put sociologically, the Christian movement as a whole was an identifiable and homogenous sect according to several Christian and non-Christian authors. There was an acute consciousness within the early churches of being a worldwide movement that saw itself connected to various groups, Jewish and Gentile, with a shared ethos and identity, who were interested in each other’s affairs, quite evident from the Pauline, Johannine, Clementine, Ignatian, and Quartodeciman letters. Paul Trebilco’s study of Christianity in Ephesus shows how different Christians groups in close proximity could certainly rub up against each other with some friction, but degrees of “commonality” still existed and Ephesian Christian assemblies were quite willing “to acknowledge the validity of each other’s claim to be part of the wider movement that we call early Christianity.” Moving into the mid-second century Justin Martyr provides evidence for tendencies of both exclusion and inclusion. Justin regarded those who denied the resurrection of the dead in preference for the immortality of the soul as false Christians, but he also believed one could associate with Jewish Christians who observed parts of the Mosaic law. For Justin it was sufficient to believe in Jesus as the crucified savior, risen Lord, and future judge for fellowship with others.
Consequently, the early church was not filled and fraught with endlessly endemic disunities and diversities. There were several commonalities shared by churches from east to west. As such, I suggest, following Hurtado, that we abandon “unity” and “diversity” for the better term “interactive diversity” to signify the multiple range of divergences and convergences across first century Christian groups reflected in the New Testament and early Christian literature. So it may be fashionable to say that there is no single theology of early Christianity available, but in the final dust up they did opt for a single theology and they called it the Holy Scriptures. In the words of Bockmuehl: “At the end of the day, when everything is said and done about the genetic vagaries of the New Testament canon’s formation, it remains an equally historical phenomenon that the church catholic came to recognize in these twenty-seven books the normative attestation of its apostolic rule of faith.”
In which case, it is a justifiable and perhaps even necessary task of biblical theology to identify the types of unity across the entire New Testament. In terms of the unity to the NTT, one could opt for a fairly minimalist version like Dunn does based on the “unity between the historical Jesus and the exalted Christ, that is to say, the conviction that the wandering charismatic preacher from Nazareth had ministered, died and been raised from the dead to bring God and man finally together, the recognition that the divine power through which they now worshipped and were encountered and accepted by God was one and the same person, Jesus, the man, the Christ, the Son of God, the Lord, the life-giving Spirit.” Schröter acknowledges that position but extends it further through a “unifying bond” or an “interpretation of reality with a centre” that includes “faith in the God of Israel as the creator of the world and human beings, in the fact that Jesus Christ represents this God with full vitality, and in the fact that he is active by the Spirit.” Or else one could opt for a cluster of convergences and coherences as Frey does: “[The] collective interpretation of the New Testament witness to the life, passion, death, and resurrection of Jesus of Nazareth is arguably the most significant reason why it is fitting to ask about the unity of New Testament theology. The common assumption of the Old Testament belief in God and, even more precisely, the testimony of the divine love of God in Christ, the eschatological tension between the ‘already-now’ and the ‘not-yet’ first present in the proclamation of Jesus and then constituted by the conscious awareness of eschatological fulfilment, or even the agreements between the Jesuanic basileianic-proclamation and the later Pauline construal of justification doctrine are further points of convergence and lines of concurrence.”
 See recently Bruno 2019.
 Bockmuehl 2006, 103.
 A point which is programmatic for Berger 1994, 4. Also, it should be noted that Ferdinand Hahn‘s (2002) New Testament Theology dedicated one volume to diversity/variety (Die Vielfalt des Neuten Testaments) and another to its unity (Die Einheit des Neuen Testaments).
 Theissen 1999, 282.
 See Balla 1997, 200-207.
 Acts 24:5, 14; 28:22; Suetonius, Nero, 16; Justin, Dial. Tryph. 108; Tertullian, Apol. 5.
 See Hvalvik 2005; Trebilco 2006; cf. Bart D. Ehrman (2003, 179-80): “The proto-orthodox were in constant communication with one another, determined to establish theirs as a worldwide communion … The proto-orthodox were interested not only in what happened locally in their own communities but also in what was happening in other like-minded communities.”
 Trebilco 2004, 716.
 Justin, Dial. Tryph. 47, 80.
 Hurtado 2013. Markschies (2015, 343-44) prefers to talk about a “plural identity” with an identity-forming center labelled as the “Holy Spirit” and pluralistic expressions in different institutions in the church.
 Bockmuehl 2006, 103 (italics original).
 See Carson 1995, 30-31. Cf. Schröter (2013 , 344) who says “a New Testament theology has to make visible the common contours within which these writings move, without thereby wanting to systematize their respective particularities more strongly than they themselves allow.” Theissen (2006, 207) refers to “the normative exposition of a religion through an interpretative summary of its canonical texts.” Hahn (2002) is quite right to dedicate one whole volume to the Unity of the New Testament: A Thematic Portrayal (Die Einheit des Neuen Testaments: thematische Darstellung). See also about unity and normativity in the early church Achtemeier 1987 and Hultgren 2004.
 Dunn 2006, 403.
 Schröter 2013 , 327.
 Frey 2007, 50-51 (trans. MFB).