The following paragraphs come from an essay I recently composed in answer to the question, "what is historical theology?"
In considering the term “historical theology,” the first matter one must confront is the question of whether we are primarily dealing with a historical or a theological discipline. If the discipline is primarily historical, then it ought to be approached as a branch of intellectual history, with the aim being to explain what individuals of the past said, and what they meant by what they said, but no more. The goal would be to gain a good understanding of the past, without “theologizing” about it in the present.
One immediately encounters a difficulty with this perspective, however. In looking at the term “historical theology,” we notice that the word “historical” is being used as an adjective, while “theology” is the noun. The discipline is not called “theological history” but “historical theology.” Therefore, I am inclined to view historical theology primarily as a theological discipline with a historical emphasis, and the historical theologian as just that: a theologian. While historical investigation is inseparable from the historical theologian’s discipline, his main business is to do theology.
Geoffrey Bromiley defines theology this way: It is “the church’s word about God in responsive transmission of the Word of God to the church.” In other words, theology is the contemplation and communication of the message of Sacred Scripture by the church, and for the church. But it is also more than that. Bromiley goes on to say that theology is also “the investigation of the church’s word about God with the intent of testing and achieving its purity and faithfulness as the responsive transmission of God’s Word in changing languages, vocabularies, and intellectual and cultural contexts.” So theology is the contemplation and communication of the message of Sacred Scripture, but it is also the study and analysis of the church’s contemplations of those Scriptures for the purpose of improving and refining her doctrines in the present.
Thus, the true historical theologian is no mere recounter of history, but is an active participant in “the church’s theological enterprise.” When he does his job well, he “shows how the church and its word, moving across centuries and continents, have come from there to here with an ongoing continuity in spite of every discontinuity…[he] offers examples of the ways in which, and the reasons why, the conformity of the church’s word to God’s Word has been achieved or compromised in the different centuries and settings…[and, he]…brings to the church of today a valuable accumulation of enduring insights as well as relevant hints and warnings.”
I would therefore suggest the following definition for historical theology: Historical theology is that branch of Christian theology which serves the church of Christ by investigating the church’s word about God’s Word as it has been expressed through time, so that she might be better equipped to develop a biblically pure and faithful system of doctrine in the present.
 Bromiley, XXVI
 Bromiley, XXVI
 Bromiley, XXVI