Unity and diversity in the NT should not be pitted against one another as if it is an either-or proposition. As one who is committed to inerrancy, I believe firmly in the unity of Scripture. Against Dunn (Unity and Diversity in the New Testament) I do think there was a normative form of Christianity in the first century though not all groups everywhere may have realized it from the very beginning. The NT is, however, part of the process of explaining and promulgating that norm so that all believers everywhere could be in unity regarding belief and, perhaps to a lesser extent, practice.
For example, though we know that Peter and Paul disagreed (because Paul tells us), we have no writings in the NT by Peter disagreeing with Paul. The end result of Paul’s disagreement with Peter is spelled out in the same narrative written by Paul and their disagreement is resolved.
One can suggest that competing groups within early Christianity, especially Jewish and Hellenistic Christians, divided from each other but since Luke portrays early Christian leaders as working together to resolve these conflicts, one must conclude that there was more unity than diversity in early Christianity. Any attempts to portray Luke as covering up such supposed differences lack evidence. Differences between Acts and Paul or James and Paul or between Gospel writers such as Matthew and Mark can be reconciled using evidence at hand and contained in the writings themselves without resorting to logical leaps, arguments from silence, or differing textual traditions as are often used to propose a stance in favor of more diversity.
Such logical leaps are exemplified in beliefs such as that development in church order and leadership implies development in core doctrines such as Christology and soteriology. Arguments from silence are evident in claims such as that since the writer of Hebrews does not mention baptism, the Lord’s Supper they must not have been aware of them.
This does not mean that there is no diversity, however. With regard to practices within the church it is clear that there was some development. This development and its expression support the unity of the NT rather than arguing against it. Since Paul writes letters that are largely occasional, this supports the notion stated above that the writings of the NT are to promote unity rather than to promote one’s views against those of other groups as is often alleged. To say that at one time churches did not have structure leadership until Paul developed the familiar form does not necessarily imply different groups pitted against one another.
Such development in practice also does not necessarily imply a development in doctrine, especially with regard to theological loci such as Christology or soteriology. Perhaps it is more appropriate to speak of theological development with regard to these theological loci as a development in expression and statement since much of what Paul, for example, does seems to be explanation of what his readers already seem to know, but only in basic terms.
Each writer does have different emphases and these varying emphases lead to diversity in the expression of doctrine. This is consistent with the prophetic tradition in the OT. Various prophets spoke of similar ideas, such as the Day of the Lord, and each expressed these beliefs in different words and different images and so each prophet is able to provide a different perspective on the same thing. Likewise, the NT writers speak of the same things in different ways.
With regard to doing theology in the context of the church, both unity and diversity must be emphasized and this will lead to tension. Christians need reassurance that the Bible is one book with ultimately one author: God. But, each human author was chosen by God to express the truth revealed to him and we must let each author speak on their own. We cannot soften warnings in Hebrews by appeal to John nor neglect or soften texts on human responsibility to find acceptance in a particular tradition. Each passage and author must be allowed to speak on his own.