Biblical Theology *is* exegesis
Good biblical theology *is* exegesis if it allows scripture to interpret scripture. IOW, no verse is *interpretively* isolated from any other verse.
"(Biblical Theology) is that part of Exegetical Theology (exegesis) which deals with the revelation of God in its (redemptive) historic continuity...Biblical Theology...discusses both the form and contents of revelation from the point of view of the revealing activity of God Himself. In other words, it deals with revelation in the active sense, as an act of God, and tries to understand and trace and describe this act, so far as this is possible to man and does not elude our finite observation. In Biblical Theology both the form and contents of revelation are considered as parts and products of a divine work." -- Geerhardus Vos, "The Idea of Biblical Theology as a Science and as a Theological Discipline"
The reason BT is an exegetical discipline is because it *traces* God's Act in Christ through the text. That *tracing* is exegesis because it involves interpretation of the text itself. What we have in Hebrews 11 is a great example of this... when the writer of Hebrews says that Jacob worshipped in faith as he leaned on his staff (Heb. 11:21), he is exegeting Gen. 47:31. He is not "building" on exegesis... he is understanding what Moses intended as Moses wrote Gen. 47. When he says that Abraham was looking for a city, he is interpreting all of the mandates and promises to Adam through Abraham and Abraham's subsequent worship and wanderings. When he says that Moses preferred the reproach of Christ, he is interpreting the Messianic underpinnings of the entire