I know what you are thinking. How profound. Talk about having a firm grasp on the obvious. Fair enough. But some of the most important keys for Bible study are hidden in plain sight. This thought is utterly crucial, and one most Bible students miss despite its transparency. To be honest, there are few things more important for biblical theology than having this fact invade your mind and establish a permanent beachhead.
Even someone who has never read the Bible can discover that the Old Testament came before the New Testament. That's what the Table of Contents is for. Yet even seasoned Christians read and study the Bible as if that observation is more a point of trivia than an indispensable clue to competent Bible study. I know of preachers who don't consider the Old Testament worthy of pulpit time. I've had many sincere Christians tell me they can't recall the last time their pastor went through an Old Testament book. The average person in the pew has been conditioned to equate the word "Bible" with "Jesus" or "the Gospels" or "the book of Revelation." Don't believe me? Ask your Christian friends this question, and see what answers you will get: "What's the Bible about?" Most answers will sound like you asked what the New Testament was about.
Ignorance of the Old Testament is a serious issue. I'd call it a hermeneutical crime. Since it came before the New Testament, it was the Bible of Jesus, the apostles, and the first Christians. The Old Testament is three-quarters of your Bible today. The New Testament books quote from it for a simple reason: New Testament theology is tethered to Old Testament theology. Since it came first, it has "coherence priority" - it is essential for understanding what follows. There isn't a page of the New Testament that doesn't reference the Old Testament in some way. Every New Testament doctrine has its roots in the Old Testament.
We like to say that Scripture must be interpreted in context. The Old Testament is the primary context for the New Testament. Without a grasp of the purpose and theology of the Old Testament, any commitment to context forfeits its authenticity.