What Is Systematic Theology?
Biblical theology provides the basis for understanding how texts in one part of the Bible relate to all other texts, according to God’s intention, which is discovered through human authors but ultimately at the canonical level. In the end, biblical theology is the attempt to think through the “whole counsel of God,” and it provides the basis and underpinning for all theologizing.
If this is what biblical theology is, then what is systematic theology? As with “biblical theology,” there are various ideas as to what “systematic theology” is. It is not necessary to delve into all of these diverse views; rather, we will simply state how we conceive of the discipline. For our purposes, we will employ the definition given by John Frame: systematic theology is “the application of God’s Word by persons to all areas of life.” 
2 Key Components
In our view, this entails at least two key components.
First, in order to apply Scripture properly, we must first interpret Scripture correctly, which requires the doing of biblical theology, as just described. This is why we contend that biblical theology is the basis for all theologizing, since we are not doing theology unless we correctly understand how the entire canon of Scripture fits together.
Second, systematic theology goes further than biblical theology, since it involves the application of Scripture to all areas of life. Systematic theology, then, inevitably involves theological construction and doctrinal formulation, grounded in biblical theology and done in light of historical theology, but it also involves interacting with all areas of life—history, science, psychology, ethics, and so on.
In so doing, systematic theology leads to worldview formation as we seek to set the biblical-theological framework of Scripture over against all other worldviews and learn “to think God’s thoughts after him,” even in areas that the Bible does not directly address. In this important way, systematic theology presents a well-thought-out worldview, over against all of its competitors, as it seeks to apply biblical truth to every domain of life.
As a discipline it is also critical in seeking to evaluate ideas within and outside the church. Outside the church, systematic theology takes on an apologetic function as it first sets forth the faith to be believed and defended, and then critiques and evaluates views that reject the truth of God’s Word. In this way, apologetics is properly a subset of systematic theology. Within the church, theology is critical by analyzing theological proposals first in terms of their fit with Scripture and secondly in terms of their implications for other doctrines. In all these ways, systematic theology is the discipline which attempts “to bring our entire thought captive to Christ” (see 2 Cor. 10:1–5), for our good as the church and ultimately for God’s glory.
How, then, should we think of the relationship between biblical and systematic theology? In our view, biblical theology is primarily a hermeneutical discipline, since it seeks to rightly divide God’s Word (2.Tim. 2:14–15). This is why the conclusions of systematic theology must first be grounded in the exegetical conclusions of biblical theology.
But then systematic theology goes further: on the basis of biblical theology it attempts to construct what we ought to believe from Scripture for today, to critique other theological proposals within the church, and also the false ideas of non-Christian worldviews, so that we learn anew to live under the Lordship of Christ.
 John M. Frame, The Doctrine of the Knowledge of God (Phillipsburg, NJ: P&R, 1987), 76.
This article is adapted from God's Kingdom through God's Covenants: A Concise Biblical Theology by Peter J. Gentry and Stephen J. Wellum.