Biblical theology is the study of how every text in the Bible relates to every other text in the Bible. It’s the study of the matrix of divine revelation. At the heart of the gospel is the person of Jesus Christ; he is the word of God come in the flesh. The nature of the gospel is such that it demands that it be at the centre of the biblical message. Biblical theology is, then, the study of how every text in the Bible relates to Jesus and his gospel. Thus we start with Christ so that we may end with Christ. Biblical theology is Christological, for its subject matter is the Scriptures as God’s testimony to Christ. It is therefore, from start to finish, a study of Christ.
Biblical theology consists in the study of what the Bible teaches as the Bible teaches it. Its focus is on the big picture of the unity of the Bible. There are two complementary approaches to biblical theology (BT). The one is synchronic, that is, it takes a part of the biblical text and opens out its theological teaching. Thus, we might have an examination of the theology of a particular book or theme. This approach, however, in order not to be fragmentary, needs the other perspective, the diachronic. This looks at the unity of the biblical message throughout the whole of redemptive history contained in the canon of Scripture.
— Graeme Goldsworthy, from an interview at Buzzard Blog