Systematic theology is based on certain assumptions.
The first assumption is that God has revealed Himself not only in nature but also through the writings of the prophets and the Apostles, and that the Bible is the Word of God. It is theology par excellence. It is the full logos of the theos.
The second assumption is that when God reveals Himself, He does so according to His own character and nature. Scripture tells us that God created an orderly cosmos. He is not the author of confusion because He is never confused. He thinks clearly and speaks in an intelligible way that is meant to be understood.
A third assumption is that God's revelation in Scripture manifests those qualities. There is a unity to the Word of God despite the diversity of its authors. The Word of God was written over many centuries by many authors, and it covers a variety of topics, but within that diversity is unity. All the information found in Scripture—future things, the atonement, the incarnation, the judgment of God, the mercy of God, the wrath of God—have their unity in God Himself, so that when God speaks and reveals Himself, there is a unity in that content, a coherence.
God's revelation is also consistent. It has been said that consistency is the hobgoblin of little minds, but if that were true, we would have to say that God has a small mind, because in His being and character, He is utterly consistent. He is the same yesterday, today, and forever (Heb. 13:8).
These assumptions guide the systematic theologian as he goes about his task of considering the whole scope of Scripture and inquiring how it all fits together. At many seminaries, the systematic theology department is separate from the New Testament department and the Old Testament department. This is because the systematic theologian has a different focus than the Old Testament professor and the New Testament professor. Biblical scholars focus on how God has revealed Himself at various points over time, while the systematician takes that information, puts it all together, and shows how it fits into a meaningful whole. This is a daunting task, to be sure, and I am convinced that no one has ever done it perfectly.
As I engage in systematic theology, I never cease to be amazed by the specific, intricate coherence of the scope of divine revelation. Systematic theologians understand that each point in theology addresses every other point. When God speaks, every detail He utters has an impact on every other detail. That is why our ongoing task is to see how all the pieces fit together into an organic, meaningful, and consistent whole.