Barry is the founder and Professor of the M.Div. program for Mindanao Grace Seminary, Philippines.
Categories of Theology
Theology is the study of God. In the broadest sense, theology refers to thinking and reasoning about God. In the Christian realm, theology is the study of God as He has revealed Himself. God has revealed Himself in creation, in His Word, and the person of Christ. When the Christian considers theology, he is looking at the general (natural) revelation of God, as well as the special (limited) revelation of God. We use the term general revelation to describe the information that is available to all people through creation. The phrase special revelation refers to the information that is limited to those who have been born again and who have the Holy Spirit to accept the truths found in Scripture.
It is an error to say “We don’t need Systematic Theology. All we need is a biblical theology.” This is not understanding what is meant by Biblical Theology. Biblical Theology is the study of progressive revelation. As we move chronologically through Scripture, we see that God is revealing more about Himself and His plan. We consider what the author knew in his setting. We would be wrong to read the revelation of the mystery of the Church back into the times of Old Testament Israel. Even those who lived during the time of Jesus had less knowledge of the Kingdom of God than we have today, given that we have a completed Canon.
“Biblical Theology is that branch of Exegetical Theology which deals with the process of the self-revelation of God deposited in the Bible.” -Geerhardus Vos 
Biblical Theology helps us to see how doctrines are revealed in time through the progression of the history of Bible writing. Included in BT would be the types and shadows in the Old Testament revealed in the person and work of Christ in the New Testament. BT shows us that the Bible is one story, being revealed in time, about God’s ultimate purpose to redeem a people for Himself.
Historical Theology (Orthodoxy):
Historical Theology should not be confused with Church History or with Biblical Theology. Historical Theology traces the development of Christian thought through various periods of Church History. We consider the Patristic (AD 100–500), Medieval (AD 500–1500), Reformation (1500–1600), Post-Reformation (1600–1700), and Modern (1700–present) time periods. We might think of Historical Theology as the history of the development of Christian thought, specifically, how doctrine was refined over time.
Practical Theology (Pastoral Theology)
This again is a case where we do not want the definition to confound us. All theology is practical. Titles are greater than the sum of their parts. Practical Theology refers to the application of theology to the life of the Christian community. Practical Theology seeks to bridge the gap between the academic world and real life. Many a seminarian has left the classroom to pastor a church and found that they feel unequipped to minister. While the parchment hangs on the wall that indicates their scholastic accomplishments, it is difficult to see the value of such learning. How does this knowledge apply when the pastor is when dealing with hurting people, problems in the church, ethical issues in society, and difficult life questions? Some have concluded that seminary was a waste of time. In most cases, it was not that their education was of no value but they were not trained on how to apply what they learned.
Systematic Theology (The Science of Theology)
Some have said that Systematic Theology is a system made by man through which they interpret Scripture. This is not true. Whereas we all have predispositions and we all should be seeking to be aware of them, Systematic Theology will reveal and correct our biases. Systematic Theology views the doctrines of Scripture as a whole system. The thoughts of God as expressed in His word, are united. Given that all Scripture is from God, and God cannot contradict Himself, then all that God says about any topic could not be in contradiction to anything else He said.
The Bible is not a book of hidden knowledge. It is a direct revelation from God. The intent of God is to be understood. When we understand the Word, we can synthesize and summarize in such a way that there are no contradictions. Proper exegesis is how we solve problems in the Scripture and come to cohesive conclusions.
Systematic Theology seeks to hold all verses to be equally inspired, equally true, and equally authoritative. Rather than a bag of jewels spilled out on the floor for us to marvel at individually, Systematic Theology helps us see the Bible as a string of pearls that are united, connected, and of one purpose. And most importantly, Systematic Theology ensures we are consistent.
 Quoted in Vern Poythress, “Kinds of Biblical Theology,” Westminster Journal of Theology, 70, (2008): 130.