Here are some key quotations from an 1894 essay by Dr. Geerhardus Vos (1862-1949), the leading evangelical theologian in the early development and popularization of the discipline of Biblical Theology. The entire essay is available in the Resources section of this site--The Idea of Biblical Theology as a Science and as a Theological Discipline (his inaugural address as Professor of Biblical Theology at Princeton Theological Seminary in 1894). Everything that follows is quoted from that address:

• The specific character of Biblical Theology lies in this, that it discusses both the form and contents of revelation from the point of view of the revealing activity of God Himself.
• In Systematic Theology these same contents of revelation appear, but not under the aspect of the stages of divine work; rather as the material for a human work of classifying and systematizing according to the logical principles. Biblical Theology applies no other method of grouping and arranging these contents than is given in the divine economy of revelation itself.
• The first feature characteristic of supernatural revelation is its historical progress. God has not communicated to us the knowledge of the truth as it appears in the calm light of eternity to His own timeless vision. He has not given it in the form of abstract propositions logically correlated and systematized. The simple fact that it is the task of Systematic Theology to reproduce revealed truth in such form, shows that it does not possess this form from the beginning. The self-revelation of God is a work covering ages, proceeding in a sequence of revealing words and acts, appearing in a long perspective of time. The truth comes in the form of growing truth, not truth at rest. No doubt the explanation of this fact is partly to be sought in the finiteness of the human understanding…gradually to be let upon us, ray after ray, and not the full radiancy at once.
• As soon as we realize that revelation is at almost every point interwoven with and conditioned by the redeeming activity of God in its wider sense, and together with the latter connected with the natural development of the present world, its historic character becomes perfectly intelligible and ceases to cause surprise.
• It accompanies in its progress the gradual unfolding of the central and objective salvation of God, and no sooner is the latter accomplished than revelation also has run its course and its voice ceases to speak.
• We must remember that the revealing acts of God never appear separated from His verbal communications of truth.
• Biblical Theology also undertakes to show how the truth has been gradually set forth in greater fullness and clearness.
• The knowledge of God has…an internal expansion, an organic unfolding from within. The elements of truth…are seen to grow out of each other, each richer and fuller disclosure of the knowledge of God having been prepared for by what preceded, and being in its turn preparatory for what follows. That this is actually so, follows from the soteriological purpose which revelation in the first instance is intended to serve.
• The Gospel of Paradise is such a germ in which the Gospel of Paul is potentially present; and the Gospel of Abraham, of Moses, of David, of Isaiah and Jeremiah, are all expansions of this original message of salvation, each pointing forward to the next stage of growth, and bringing the Gospel idea one step nearer to its full realization.
• So dispensation grows out of dispensation and the newest is but the fully expanded flavor of the oldest.
• The various stages in the gradual concentration of Messianic prophecy, as when the human nature of our Savior is successively designated as the seed of the woman, the seed of Abraham, the seed of Judah, the seed of David, His figure assuming more distinct features at each narrowing of the circle—what are they but disclosures of the divine counsel corresponding in each case to new realities and new conditions created by His redeeming power?
• For, although the development of the root may be slow and the stem and leaves may grow almost imperceptibly, there comes a time when the bud emerges in a day and the flower expands in an hour to our wondering sight. Such epochs of quickened revelation were the times of Abraham, of Moses, of David, and especially the days of the Son of Man.
• This progress, moreover, increases in rapidity the nearer revelation approaches to its final goal.
• It is this triune God who here reveals Himself as the everlasting reality, from whom all truth proceeds, whom all truth reflects, be it the little streamlet of Paradise or the broad river of the New Testament losing itself again in the ocean of eternity.
• All the separate lines along which through the ages revelation was carried have converged and met at a single point. The seed of the woman and the Angel of Jehovah are become one in the Incarnate Word.
• For, God having chosen to reveal truth through human instruments, it follows that these instruments must be both numerous and varied adaptation to the common end.
• God's method of revelation includes the very shaping and chiseling of individualities for His own objective ends. To put it concretely: we must not conceive of it as if God found Paul “readymade," as it were…
• …God chose Paul from the womb, molded his character, and gave him such a training that the truth revealed through him necessarily bore the dogmatic and dialectic impress of His mind.
• The human is but the glass through which the divine light is reflected, and all the side and angles into which the glass has been cut serve no other purpose than to distribute to us the truth in all the riches of its prismatic colors.
• Biblical Theology, rightly defined, is nothing else than the exhibition of the organic progress and supernatural revelation in its historic continuity and multiformity.
• The Bible contains…, if I may so call it, a divine philosophy of the history of redemption and of revelation in general outlines.
• Systematic Theology endeavors to construct a circle; Biblical Theology seeks to produce a line. I do not mean by the use of this figure that within Biblical Theology there is no grouping at all. The line of which I speak does not represent a monotonous recital of revelation, and does not resemble a string, even though it be conceived as a string of pearls. The line of revelation is like the stem of those trees that grow in rings. Each successive ring has grown out of the preceding one.
• Biblical Theology will also demonstrate that the fundamental doctrines of our faith do not rest, as many would fain believe, on an arbitrary exposition of some isolated proof-texts. It will not so much prove these doctrines, as it will do what is far better than proof-make them grow out organically before our eyes from the stem of revelation.

Related Resource - Available for Download

The Idea of Biblical Theology by Geerhardus Vos
Dr. Geerhardus Vos (1862-1949) was the most influential evangelical Bible scholar in the early development and popularization of the discipline of Biblical Theology (the counterpart to Systematic Theology). This essay was his inaugural address as Professor of Biblical Theology in Princeton Theological Seminary on May 8, 1894. Although it's not easy reading, it does help to clarify the distinctives and values of Biblical Theology.