Systematic theology is an exposition of truth about God and his purpose and interaction with humanity based both on scripture and what can be known or surmised from general revelation (e.g., psychology, science), as it is probed from a person’s contemporaneous perspective and synthesised into a coherent logical system to address the comprehensive life questions facing the world in which he currently lives.
This definition is rather off the cuff and may have some weaknesses, but two points worth emphasising is that 1) systematic theology employs the logical categories and priorities of the contemporary world, and 2) systematic theology is not solely based on scripture (special revelation), but also on knowledge culled from other sciences (general revelation).
This may be compared with biblical theology. Biblical theology is an exposition of biblical truth according to the Bible’s own terms and categories and urgencies. It is limited in terms of its basis to the biblical text itself apart from the sciences, although the disciplines of archaelogy, history, sociology, etc., may shed light on our understanding of the text.
Biblical theology is descriptive, while systematic theology is prescriptive. Thus, biblical theology tells us what they believed back then, while systematics tells us what we ought to believe. Of course, the biblical writers wrote with a view that their theology was something which should be imported and imposed world-wide for all time and for all people, and thus, the prescriptive nature of the text cannot be gainsaid. Yet, the task of biblical theology as a discipline is to sort out their beliefs back then precisely so that systematic theologians can put it into modern categories and logical systems to prescribe to moderns how they should now live.
On a side note, one should distinguish that systematics is an exposition of what one should believe, while apologetics is a defence of that exposition.
Two further observations are in order. First, a person can take a biblical theology approach to individual books and corpuses of the Bible and produce such a thing as a “biblical theology of the Pastoral Epistles” or a “biblical theology of the Pentateuch.” The same is true of the two testaments. There is such a thing as a “biblical theology of the Old Testament” and a “biblical theology of the New Testament.
Of course a proper biblical theology takes into account the entire canon. One certainly cannot do a biblical theology of the Johannine writings and call it simply a biblical theology. This is especially true of a biblical theology of the New Testament; one can produce a biblical theology of the New Testament, but without the Old Testament, it cannot truly be called a biblical theology.
Secondly, biblical theology can be organised in various schema and still be considered a biblical theology. Biblical theology does not require a sequential or a synchronic presentation in order to qualify as biblical theology. In Gerhard Hasel’s Old Testament Theology: Basic Issues in the Current Debate, examples of these various schemas are outlined. He discusses 1) Dentan’s paradoxical attempt to present a biblical theology in traditional dogmatic categories, 2) the Genetic-Progressive presentation, 3) the Cross-Section presentation, 4) the Topical presentation, 5) the Diachronic presentation, 6) the “Formation-of-Tradition” presentation, and others (28-114). My point here is that the no matter how you organise or present the “stuff” of biblical theology it will still be recognisable as a biblical theology, so long as it is descriptive of the biblical text and retains its same categories and priorities.
In passing, one should note that the older systematic theologies cannot be relied upon for accurately distinguishing between systematics and biblical theology for the simple reason that biblical theology is a relatively young discipline, with relatively few biblical theology works being published prior to the 20th century, and only blossoming into full flower in the last half of the 20th century.
This concludes my discussion of the differences of biblical theology and systematics. Here are some supporting citations and thoughts from important works (all EMPHASES belong to me):
Waltke (An Old Testament Theology): Dogmatic (systematic) theologians serve the church best when they rely on orthodox biblical theology for explications of Scripture from which they frame abstract universal propositions in accordance with a coherent system APPROPRIATE TO THE CHURCH’S CONTEMPORARY SITUATION (31).
Waltke: [Famed systematician Charles] Hodge failed to realize adequately that the biblical writers had their own priority of ideas and coherency of thought and that the biblical theologian aims to honor that priority and arrangement by tracing the trajectory of the themes that are found in and run through the books (51).
Waltke: Systematic (dogmatic) theologians present the Christian message to the CONTEMPORARY WORLD. They draw the impetus for organizing this message from outside the Old Testament. John Calvin…organized his material according to the four divisions of the Apostle’s Creed. Philip Melanchthon organized his theology according to one book of the Bible, Romans. Since the seventeenth century, theologians typically employed philosophical categories derived from Greek thought, such as Bibliology, (the study of the Bible), hamartiology (the study of sin), penumatology (the study of the Spirit), and so on (64).
Waltke: Biblical theologians differ from dogmaticians in three ways. First, biblical theologians primarily think as exegetes, not as logicians. Second, they derive their organizational principle from the biblical blocks of writings themselves rather than from factors external to the text. Third, their thinking is diachronic—that is, they track the development of theological themes in various blocks of writings. Systematic theologians think more synchronically—that is, they invest their energies on the church’s doctrines, not on the development of religious ideas within the Bible (64).
I. Howard Marshall (New Testament Theology): “[Systematic theology] is intended to DESCRIBE a theology that is not so much a description of what Christians believe as rather what they OUGHT to believe” (43).
Gerhard Hasel (Old Testament Theology): In detailing the birth of the discipline of biblical theology Hasel writes, “…Johann Philipp Gabler…made a most decisive and far-reaching contribution to [biblical theology]…. Gabler’s famous definition reads: ‘Biblical theology possesses a historical character, transmitting what the sacred writers thought about divine maters; dogmatic theology, on the contrary possesses a didactic character, teaching what a particular theologian philosophizes about divine matters in accordance to his ability, time, age, place, sect or school, and other similar things” (17).
Hasel: “Biblical theology is not aiming to take the place of or be in competition with systematic theology as the latter expresses itself in the form of system building based on its own categories either with or without the aid of philosophy” (33).
Millard Erickson (Christian Theology): “So we propose a more complete definition of theology: that discipline which strives to give a coherent statement of the doctrines of the Christian faith, based primarily upon the Scriptures, placed in the context of CULTURE IN GENERAL, worded in a CONTEMPORARY IDIOM, and related to ISSUES OF LIFE… (21).
Erickson: “Theology must also be CONTEMPORARY . While it treats timeless issues, it must use language, concepts, and though forms that make some sense in the context of the present time…. It is not merely a matter of using today’s thought forms to express the message. The Christian message should address the questions and the challenges encountered TODAY” (24).
Erickson: In distinguishing two competing approaches to biblical theology with systematic theology, Erickson writes, “We might today call this the distinction between descriptive biblical theology and normative biblical theology. Note, however, that neither of these approaches is dogmatics or systematic theology, SINCE NO ATTEMPT IS MADE TO CONTEMPORIZE OR TO STATE THESE UNCHANGING CONCEPTS IN A FORM SUITABLE FOR OUR DAY’S UNDERSTANDING” (24-25).
D.A. Carson (“The Role of Exegesis in Systematic Theology” in Doing Theology in Today’s World, Woodbridge and McComiskey, eds.): Biblical theology is bounded in two ways. “First, its subject matter is exclusively biblical…. Second, it organizes its subject matter in ways that preserve corpus distinctions. It is less interested in what the New Testament or the Bible says about, say, the sovereignty of God, than it is in what Paul (or Isaiah, or John) says about this subject…. This means, in turn, that biblical theology is organized chronologically, or better, salvation-historically…--both within any one corpus…and from corpus to corpus (45).
Carson: “Systematic theology, then is Christian theology whose internal structure is systematic; i.e., it is organized on atemporal principles of logic, order, and need, rather than on inductive study of discrete biblical corpora (45).
Since Carson is not so interested in contrasting systematics with biblical theology, some of his statements about systematics which actually do contrast with systematics need to be elucidated. For example, when he emphasises that systematics poses atemporal questions (45-46), this implies that biblical theology addresses the priorities of ancient Israelites and early Christians which may not necessarily coincide with those urgencies facing humanity in other cultures or time spans.
Carson rightly discerns that biblical theology is imminently concerned with the theology of a given corpus (the doctrine of land in Joshua, resurrection in the Johannine literature, the kingdom of God in Matthew, etc.). He also rightly discerns that there is such a thing as unity and diversity between the corpora, and that one corpus might take a different perspective on any subject matter, and that good biblical theology must ultimately integrate these different perspectives into a unified theology. It is important to note, however, that the synthesis of a given theological issue in the various corpora is neither the product nor the task of systematic theology. Such a synthesis is still entirely the domain of biblical theology. Now, if the results of such a synthesis were to be applied to the contemporary and eternal questions which dog humanity, and re-cast in logical hierarchies and categories, as well as integrated with whatever other knowledge can be culled from the realm of general revelation, then the final result would be systematic theology.