The word theology is a transliteration of the Latin theologia which itself is a transliteration of the Greek. Richard Muller helpfully develops the etymology of how ‘theology,’ the word, came into usage among Christians, and in particular how that took form among the Protestant Reformed Christians during the 16th and 17th centuries. Let me quote Muller at some length on this in two separate chunks, and then I will close with some final reflection.
The word theologia is of Greek origin, taken over into Latin, and then borrowed or adopted by the fathers of the church from gentile writers. According to Aristotle and Cicero, the poets were to be called “theologians” because they spoke of the gods and of “divine things.” Thus, by adaptation and extension of the classical usage, Lactantius refers in his De ira Dei to those who know and worship God rightly as theologi and to their knowledge as theologia. Early on, moreover, Christians referred to the apostle John as Theologus, “the Theologian,” in titles added to the Apocalypse. Alsted adds to this the fact church fathers, like Nazianzus, were called Theologus because they wrote about and defended the doctrine of the Trinity.
Like so much in the Christian heritage, in the ‘fullness of time’ as it were, us Christians have retexted grammar provided for by the classical Greek thinkers. You could imagine though, that as the Protestant Reformation took place, Christian Humanist movement that it was (i.e. ad fontes ‘back to the sources’ e.g. the Biblical text in its original languages and to the Patristics/Church fathers), the Protestant scholastics almost stumbled over the appropriation of the language theologia or theology; because in their minds it was too closely associated with Hellenistic philosophy, paganism, and what had caused so many of the problems that they were protesting against within the Roman Tridentine Catholic system.
The fact that the term theologia itself is not a biblical but an ancient pagan term cause the Protestant scholastics some brief anxiety. After all, the Reformation was, if nothing else, a profoundly biblical movement, zealous to avoid anything in religion that could not be justified from Scripture and careful, particularly in its first several decades, to formulate its theology upon the text of Scripture and to avoid the use of classical as well as medieval sources. The classic use of the term theologia by Aristotle and Cicero was not easily assimilated by Protestant system either on the basis of the ancient inscription to John as Theologus or on the basis of the usage of the fathers of the church, since pagan “theology” neither had access to supernatural or special revelation nor was capable of a proper use of reason in discerning the truths of natural revelation. What Christians call theology, by way of contrast with the ancient pagan usage,
is a science of divine things … which treateth of God, nor according to human reason, but divine revelation, which showeth not only what God is in himself, but also what he is toward us; nor doth it only discusse of his nature, but also of his will, teaching what God expecteth of us, and what we should expect from God, what we should hope for, and what we should feare. [Du Moulin, Oration in Praise of Divinity, 10-11.]
Some further, preferably biblical, justification of the term was desirable. Turretin resolves the problem by making a distinction between the term theologia and its significance:
The simple terms from which it is composed do occur there, as for example, logos tou theou and logia tou theou, Rom. 3:2; I Pet. 4:10; Hebrews 5:12. Thus it is one thing to be in Scripture according to sound (quoad sonum) and syllables, or formally and in the abstract; and another to be in Scripture according to meaning (quoad sensusm) and according to the thing signified (rem significatam), or materially and in the concrete,; “theology” does not appear in Scripture in the former way, but in the latter. [Turretin, Inst. theol., I.i.2.]
Theologia, then, indicates heavenly doctrine (doctrina coelestis) and has, in addition to the scriptural references to logia tou theou, words of God, a series of scriptural synonyms: “wisdom in a mystery (1 Cor. 2:7), “the form of sound words” (2 Tim. 1:13), “knowledge of truth according to piety” (Titus 1:1), and “doctrine” (Titus 1:9). Again, the thing signified by the term is discussed throughout Scripture.
Just as with everything else in the Protestant Reformation words themselves were put to the test by way of the canon of Holy Scripture. The word ‘theology’ passed the test because it signifies something that truly is grounded in the reality found in the text of Scripture as it finds its reality and order of being from Godself. In a denotative or generic sense, just as with its origin, the word theologia and theologi (i.e. theology’s practitioners) can be used very generally with reference to anyone who studies a particular conception of god; however, when Christians use the term we understand that the reference of this word is to the Triune God, and that Jesus Christ himself is truly Theologus pro nobis (The Theologian for us).
 Richard A. Muller, Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy, ca. 1520 to ca. 1725 (Grand Rapids, Michigan: Baker Academic, 2003), 152.
 Ibid., 153.