What is Theology?
Originally, the term meant the study of God. Most people today understand it to mean all that Bible teaches on any given subject.
What is systematic theology?
This is what most people understand by the term "theology". All that the Bible teaches on any given topic is systematized into a coherent statement that is consistent with all that the Bible teaches on every other subject.
What is biblical theology?
Biblical theology is not as topical as systematic theology. It studies the growth of our understanding of theology throughout the eras of biblical history. The assumption here is progressive revelation.
What is historical theology?
Historical theology is the same as biblical theology with the exception that it applies to all of church history, not just biblical history.
What is book theology?
Book theology is where one investigates the teaching of a specific book; e.g. the theology of First and Second Peter.
What is author theology?
This is when one studies the specific theological understanding and emphases of a person; i.e. the theology of Paul, or James, etc.
Why is theology necessary?
Theology is necessary for a variety of reasons. The first is the form in which God's revelation comes to us. Sadler writes:
Another point of the greatest importance requires notice both the Founder of the Religion in His discourses and the writers of these letters to the members of His Church take it for granted that the various members of the Church had already in their hands an inspired volume; i.e. the Old Testament. The way in which the Old Testament is treated is very remarkable and at first sight somewhat perplexing. It is recognized as the word of God indeed before the first books of the New Testament were written, it was the only written word of God to the Christians. All of it is supposed to be given by inspiration of God. It is referred to continually as declaring the will of God; it is assumed to be full of principles which are eternal in their application, and yet certain parts of it are supposed to be abrogated. This then is the form which the Christian Scriptures take, not the form of a methodical treatise in which we can find all that pertains to any particular doctrine digested into certain chapters or sections, but such a form that if we wish to ascertain the mind of God on any particular subject we must gather together scattered statements, intimations, hints, illustrations as they present themselves in the New Testament Scriptures, not for a moment forgetting that these New Testament Scriptures at every turn witness to the abiding authority of certain older Scriptures equally given by inspiration of God. source