In the latest 9Marks Newsletter, a number of theologians answer this question. Here is Graeme Goldsworthy's answer:
If a pastor does not understand or teach biblical theology, the members of the congregation are likely to suffer thus:
1. Congregations will not understand the unity of the Bible or the progressive nature of revelation. They will fall prey to those proclaiming the disunity of the biblical message; and a fragmented Bible cannot be recognized as the inspired word of God.
2. Congregations will not understand the centrality of Christ for interpreting Scripture and the meaning of life in our world. Recourse to people and events—particularly those of the Old Testament—will be valued mainly for their exemplary lessons, and not for their typological contribution to understanding the person and work of Christ. They will not see that Christ in his gospel is the interpreting principle for scripture and, indeed, for all reality.
3. Grace will be eroded by legalism. Preaching that principally points to the examples of Bible characters leads almost inevitably to legalism since the connection with the gospel of grace will be clouded or even completely lost.
4. The application of Bible texts will often be short-circuited. The Bible is reduced to a lucky-dip of texts all of which are perceived as standing in the same essential relationship to the Christian believer, and the progressive nature of biblical revelation in salvation-history is ignored.
5. The presuppositions of the New Testament in portraying Christ as the fulfiller of the Old Testament will be overlooked so that the fullness of Christ’s person and work is undermined. Teaching from the Old Testament is particularly at risk.
6. The doctrinal formulations of the church will be seen as less important in that their relationship to the progressive revelation of the Bible will not be evident. Biblical theology and doctrine work together for a robust understanding of God and his purposes for his people and the world.
Graeme Goldsworthy is the visiting lecturer in hermeneutics at Moore Theological College in Sydney, Australia. He is the author of According to Plan (IVP, 1991) and Preaching the Whole Bible as Christian Scripture (Eerdmans, 2000).
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Check out the other answers to this question, as well as other essays on biblical theology.