The Bible is God's revelation to us, and the Bible was written by people like us. One thing this implies is that because the Bible was written by a bunch of different people using different languages and genres, there is an inherent diversity to the Bible. Different people in different times and settings write differently. This may seem obvious, but we tend to forget about the Bible's inherent diversity whenever we focus more on defending the Bible to the secular world than on absorbing its stories for ourselves.
Another thing this implies is that our ability to understand the Bible requires us to have some level of fluency with the language and literary conventions of the Bible. Thankfully, I have an English translation of the Bible, because I can't read anything in Hebrew. Similarly, I'm grateful that I know how to read poetry. So, when I encounter a metaphor in the Psalms like the one from Psalm 33 about God gathering the sea into jars, I get that this means something about God's control and care of creation not that there is some giant warehouse full of sea-filled jars out there.
The subject matter of the entire Bible is God and his redemption of us and all creation. The Bible is God's revelation of himself and his cosmic redemption plan. The Holy Spirit worked through all of those people who wrote the Bible, and this gives the Bible its inherent unity. The Holy Spirit is the primary author of the Bible. So, while the subject matter of the entire Bible is of God and redemption, the ways in which this is communicated to us vary by things like author and genre.
One implication of this is that this revelation is historical. We can go further than this and state that the center or culmination of this history is the death and resurrection of Jesus. Scripture is revelation of God's redemption. This revelation is historical, and the centerpiece of this history is Jesus' death and resurrection. This means that the revelation of redemption in the Bible progresses towards Christ. This progression does not happen in some sort of uniform or smooth curve. Instead, this progression happens through the various genres, writers, and settings of the Old Testament that we see referenced throughout the New Testament.
As a result, we as Christians read the Old Testament using the light of the New Testament just as Jesus and the New Testament authors themselves did. We do this, because we believe with them that there is this intentional continuity and meaning throughout the Bible that centers on Jesus. We believe that the whole Bible is about God's cosmic redemption plan. This plan is revealed to us progressively and diversely.
An implication of this is that when we encounter something in the Old Testament that could be read or interpreted as irrelevant or even contradictory to redemption in Christ, we should consider that reading to be illegitimate. Instead of reading that particular passage illegitimately, we should look for an interpretation that is in concert with the Bible's continuous focus on redemption that culminates in Jesus. Trying to read the Old Testament without the light and vantage point we have from the New makes us more likely to misinterpret these passages. That being mentioned, however, we must remember that the Bible's revelation of God's redemption plan is progressive and diverse.
The Old Testament's salvation by faith in God's promises is continued and ultimately fulfilled in the New Testament or new-covenant's salvation by faith in Jesus. Jesus is the fulfillment of God's promises, and Hebrews 11:1-12:2 makes this abundantly clear. As Hebrews shows us, we should not ignore or even flatten out the differences between the Old and New Testaments even as we recognize their essential and intentional unity.