Biblical theology helps us get our arms around the big picture that ties together everything from Leviticus to Esther, and we see how Amos, John, Romans, and Revelation fit, too. Knowing what the forest looks like enables understanding of the individual trees. If we are to preach the whole counsel of God, we need biblical theology.
What sets the agenda in your preaching? Nehemiah really is not about the building program some church wants to initiate, and the Psalms are not in the Bible for amateur psychotherapists to explore their inner depths from the pulpit. Has God communicated His agenda in the Bible’s big story—its overarching message? Does the Bible’s big story set the agenda for your preaching or does something else drive it? If we are going to understand God’s purposes, which are revealed in the Bible, we need biblical theology.
Biblical theology pushes us to understand the contribution individual books of the Bible make to the Bible’s big story. We might call the Bible’s big story its metanarrative. However we describe it, the point is that the whole Bible fits together to tell us God’s revealed story of where the world came from, what is wrong with it, what He is doing to fix it, where we fit in the program, and what we can expect in the near and distant future. But there is a greater end to all this information: God is revealing Himself to us. We need biblical theology to know God. Knowing God fuels worship. Biblical theology is for worship.
Thinking in terms of biblical theology really boils down to reading the Bible in context—not just the near context of the phrase, sentence, paragraph, the wider passage, or the individual book but also the context of the whole canon. If we do not read the Psalms in the context of the canon of Scripture, we can make the Psalter’s abstract statements mean almost anything we want (especially if we foolishly ignore the superscriptions of the Psalms). If we do not read Samuel and Kings in light of Deuteronomy, we will not understand the way the narrator of those books is subtly condemning and commending people by the ways he notes what they did. If we know Deuteronomy, we will know whether God’s law has been kept or broken.
Biblical narrators cast characters as negative, positive, or ambiguous as they rehearse a character’s deeds. They relate what the characters did, and they render unspoken judgments, for instance, by showing deeds of disobedience, even if the broken law is not restated as the disobedience is narrated. Moving from the Old Testament to the New, if we do not read the Gospels and the Epistles in light of the Law and the Prophets, we will not understand how what God has accomplished in Jesus is the fulfillment of everything in the Law, Prophets, and Writings (cf. Luke 24:44). We must read each piece of the Scripture in light of the canonical whole, which is to say, we must do biblical theology.