The advantages of studying theology systematically are several.
1) We thus ascertain all that nature and the Scriptures teach on each point.
2) We compare all these teachings one with another and are enabled to define their mutual limitations.
3) We are brought face to face with the fact that our knowledge is bounded by God’s revelation, and are led to acknowledge it as its source.
4) We are consequently warned not to omit any of the truth ascertained from any source, nor to add to it anything not properly embraced therein. A departure from this rule will lead into inevitable error.
5) The harmony, and consistency, which will be found in all God’s teachings, from whatever source we may draw them, will become conclusive proof of the divine origin of revelation. This will result, not only from a comparison of what Reason and Nature teach, with the revelations of God’s Word, but of each of the several books of the Bible with the others, and especially of the body of the Old Testament as one book, with that of the New Testament as another.
6) We are thus led to value each of the doctrines of the word or God. Each is true. Each has been revealed that it might be believed. We cannot therefore omit any one, because of its forbidding aspect, or seeming unimportance, or its mysterious nature, or its demand for great personal sacrifice, or its humiliating assertions, or requirements, or the free terms upon which it assures of life and salvation.
– from An Abstract of Systematic Theology, 8.