It is very important to remember that there is historical progression in Scripture. The Bible is a historical book that records stories from the beginning of the world to the 1st century AD (and beyond, if you think of the prophecies). But the Bible isn’t a regular history book; it is what we call redemptive history. That means the Bible contains the inspired history of God’s redemption of his people. Hand in hand with this truth is the fact that there is also a progression of God’s revelation in Scripture. As time marches on, God slowly but surely reveals more of himself to his people. There is progression in God’s revelation from Genesis to Revelation. These are some assumptions of biblical theology gleaned from Scripture.
Richard Lints does a nice job explaining the importance of bibilcal theology in chapter seven of The Fabric of Theology. He wrote this in 1993, so it might be a bit more nuanced today, but it is still a helpful quote:
“A theological framework that fails to capture the ‘organic unity’ in this flow of redemption and revelatory history will likely be guilty of unnecessary abstraction from the text of Scripture. Normally one of two errors is committed by modern evangelical interpreters who take this route. Some overstress the continuity between the epochs (a la theonomy); others overstress the discontinuity of the epocs (a la dispensationalism). I concur with Edmund Clowney’s observation that ‘modern dispensationalism rightly recognizes that there are great divisions in the history of redemption; it errs in failing to grasp the organic relation of these successive eras, as the developing manifestations of one gracious design.'”
“The theonomic movement rightfully recognizes the underlying unity of the Old and New Testaments but fails to notice the organic progression present between the two Testaments. While I cannot settle all of the exegetical questions that arise in this context, I do think it is important to remember that an interpretive framework built on the assumption of divine authorship in history will seek to make clear the organic relations among the divergent epochs of the Bible. This need not result in a bland uniformity or essential contradictions of principles across epochs; rather, it should help the reader to see the overarching purpose progressively revealed through the different epochs of the Scriptures.”