The question is indeed exceedingly broad, largely historical, and liable to be closed, perhaps rightly.
But just quickly and generally, in addition to Shane's interesting answer, I would first say that the attachment of the Old Testament to the Gospel and the doctrines of Paul was not obvious nor a foregone conclusion. Indeed, it was, in my own view, very unfortunate, weighing down with confusion, ritual, and patriarchal bile an otherwise Axial Age message, as Karl Jaspers defined it, alongside those of Buddha, Confucius, and Socrates.
The reason was in part the very high esteem in which the Roman world held all things ancient, including the Jewish scriptures. Antiquity itself was a form of validity and legal sanction. And the absence of a particular historical "authorship" preserved the interwoven texts against skeptical attack. As a "nouveau enthusiasm," on the other hand, Christian cults appeared disorderly, artificial, and garish. Cementing hermeneutical relations with the ancient texts was a crucial evolutionary adaptation.
Of all the texts at last included into the New Testament, Revelations was the most highly debated, and again, in my view, unfortunate. It makes a jarring addendum, following the Jewish wars and the destruction of the Second Temple, to the universalist advances of the Gospels and Pauline epistles, with its crypto-revolutionary, horror-movie denunciations of the Roman system, enemies within, and earthly law.
Then the American period of interpretive democracy. It is essential to a "sacred text" that it remain alive and avoid historical closure by ceaselessly generating interpretations out of contradictions, parables, and complexities.. and that it "regenerate" societies and administrations capable of so preserving the text. For better or worse, the Bible in the New World underwent the textual equivalent of the Cambrian Explosion.
Geography gave rise to rapid speciation. From the moment the Puritans expelled Roger Williams, the spacious land enabled dozens of break-away denominations and sects to flourish, creating their own idiosyncratic interpretations unconstrained by violent authority... or by rational debate. While some were utopian, many American denominations not surprisingly seized upon the wretched old patriarchal narratives and the fear-mongering of Revelations, a legacy still roiling our political system.
Sorry, little more than an opinionated sketch, but given the likelihood the question will be closed anyway, why not? A more specific answer to your question would entail a veritable history of church doctrines.