Prefatory note: This post will be a little longer than normal, but it contains important material that will serve as a reference point for future entries. Special thanks to Bobby Grow for the Vanhoozer quote.
Most Christians have at some point probably heard the name Thomas Aquinas. This is
especially true in western Christianity (i.e. Roman Catholicism and Protestantism) in which this thirteenth-century Dominican monk looms particularly large. However, what many of these Christians (at least among Protestants) probably do not realize is the extent to which Aquinas’ influence extends, not only over their own church but also over the entire sphere of their lives, from politics to ethics, from national law to private morality, from just war theory to marriage and family. The purpose of this post is not to get entangled in all of these details (for an accessible introduction read Aquinas for Armchair Theologians by Timothy Renick) but rather to uncover the impact that Aquinas still has on our understanding of God but that for many Christians remains hidden and unknown. It is an understanding of God often called “classical theism,” and it functions as the interpretive lens through which we tend to read the Bible and construct our theology. Due to its pervasive influence, we typically are not aware of how much it is coloring our way of interpreting Scripture. Rather, like a fish that does not realize that it is wet because it has always been surrounded by water, we swim blissfully ignorant in the sea of classical theism without recognizing where it came from, why it may be problematic, and whether there are any other alternatives.
Here is how Kevin Vanhoozer helpfully explains the genesis of classical theism and culminating in Thomas Aquinas:
“Evangelical theologians live in the house that Thomas built.” While this is too simplistic, it is true that most evangelical theologians embrace some form of classical theism of which Thomas Aquinas was the leading medieval exponent. Classical theism began when Christian apologists of the second century somewhat necessarily used then dominant concepts of Greek philosophy to commend the faith, and the Scriptures, to the cultured despisers of religion. Theists define God as being of infinite perfection: all-holy, all-powerful, all-knowing, and everywhere present.
Classical theism refers to what has long been presumed as a synthesis worked out in the ancient and medieval church between biblical Christianity and Greek philosophy, and in particular between “God” and Aristotle’s notion of the “Unmoved Mover” (or Uncaused Cause). The Unmoved Mover is a perfect being: self-sufficient, eternal, and pure actuality (actus purus). From the latter — that God has no unrealized potential — Aristotle deduced that the Unmoved Mover must be immutable, because any change would be either for better or worse, and a perfect being is already as good as it can, and will for ever, be. God must not therefore have a body, because all bodies can be moved, so God is not material but immaterial. So: God sets the world into motion yet nothing moves God.
Thomas Aquians did not appropriate Aristotle’s Unmoved Mover wholesale. He realized that philosophy (a.k.a. “natural theology”) takes us only so far. Reason yields knowledge concerning the world of nature and, by extension, its Creator, but only revelation gives knowledge of the realm of grace and hence of the Son and Spirit. Nevertheless, by employing Aristotelian categories (e.g. substance, form, essence) and by conceding some knowledge of God to reason alone, the die of classical theism was arguably cast.
The first part of Aquinas’s Summa discusses the “one God (de Deo Uno) and treats themes accessible to natural reason — doctrines that would be held in common by Christians, Jews, and Arabs alike. Here we find discussions of God’s existence, unity, nature, and attributes. Aquinas treats the “three persons” (de Deo Trino) second, when he turns to the truths of revelation. He consequently presents the divine attributes before he even begins referring to the Incarnation and passion of the Son; in brief, he has been read as thinking about God apart from the gospel. 
Notice this critical final paragraph: Aquinas had largely thought out his notion of God in conjunction with Aristotelian philosophy and prior to his consideration of Jesus Christ as God’s definitive self-revelation. As a consequence, his view of God, at least as initially developed in the Summa, was one that could easily be affirmed by all monotheistic faiths and even some forms of pagan philosophy! In short, his initial framing of the doctrine of God was decidedly not Christian. If we ask why he did this, we must consider his particular view of nature and grace that reinforced his confidence in natural human reason to arrive at a knowledge of God:
The attitude of Thomas is best understood in its historical contrast to that of Augustine. Although Aquinas sought at every turn to harmonize his teaching as far as possible with Augustine’s…the difference between them was fundamental. His predecessor never seems to have freed himself entirely from the Manichaean conviction of cosmic evil. His mystical doctrine of the fall extended the effects of a cosmic evil will to nature itself, so that all nature is corrupt, not only human nature. Reason in man remains, but is helpless since it cannot operate apart from the will, which has lost its freedom through sin. There is consequently a sharp division between the realm of nature and the realm of grace, such as renders it impossible to explain how man can be regenerated through grace without apparently destroying the continuity of his own endeavour, and equally impossible to maintain that he can attain any knowledge of God or of divine things through knowledge of the created world…
The teaching of Aquinas contrasts with that of Augustine on every point which we have mentioned, representing a kindlier view both of man and of nature. The will is free, and the natural desire for the good persists despite sin. Aquinas is more definite than Augustine that reason itself is impaired by sin. But he holds that it can be used, and that we must follow our reason as far as it will take us. Grace and revelation are aids which do not negate reason. Here as everywhere nature itself demands supernature for its completion, and the provision of divine grace meets the striving of human nature in its search for the ultimate good, this quest being itself due to the gracious moving of God. In so far as they are, created things are good, and in so far as they are and are good, they reflect the being of God is their first cause. The natural knowledge of God is therefore possible through the knowledge of creatures. Not only so, but there is no human knowledge of God which does not depend on the knowledge of creatures…
As the first active principle and first efficient cause of all things, God is not only perfect in himself, but contains within himself the perfections of all things, in a more eminent way. It is this that makes possible the celebrated analogia entis, whereby the divine nature is known by analogy from existing things…It is a fundamental principle of Aquinas that every agent acts to the producing of its own likeness. Every creature must accordingly resemble God at least in the inadequate way in which an effect can resemble its cause…Names which are derived from creatures may therefore be applied to God analogously…The application of them must, however, respect the principle of “negative knowledge,” which is observed by most thinkers of the millennium following Plotinus when speaking of the transcendent. Plotinus had maintained that anything whatever could be truly denied of the divine being, and also that whatever we affirm, we must forthwith affirm the opposite (Enneads V). Aquinas maintains that we can know of God’s essence only what it is not, not what it is, but that this is properly knowledge of God. 
I want to conclude this post with the following observations. I think that most Christians are unaware as to how deeply their view of God has been shaped primarily by kind of classical theism espoused by Aquinas. When we begin, as for instance the Westminster Confession does, by defining God on the basis of his essence and attributes that we derive by reasoning from what God must be in contrast with his creation (i.e. a negative knowledge of God based on analogies in creation such as aseity, simplicity, immutability, impassibility, omniscience, omnipotence, etc.), we are adopting a doctrine of God that does not so much derive from God’s own self-revelation in Christ but from a synthesis worked out long ago in conjunction with the speculations of Greek philosophy such as Aristotle and Plotinus. Whereas Aquinas was comfortable with this approach because of his confidence in the ability of natural human reason to attain a limited but nevertheless true knowledge of God, I want to declare a loud Barthian “No!” because of my allegiance to a more Augustinian (not Manichaean but biblical!) understanding of the radical effects of sin on all human knowing, a condition that can only be overcome by the disruptive and recreative grace of God in Christ. The problem, however, is that many Christians who simply assume that classical theism is the biblical view of God do not realize how little of it actually coheres with how God has revealed himself in Christ as attested by Scripture. It is my hope that while this post may not clarify everything in this regard, it will at least cause us to stop for a moment and reflect on whether our own reading of Scripture is not significantly influenced by the conception of God articulated by Aquinas.
 Kevin J. Vanhoozer, ‘The triune God of the Gospel’, in The Cambridge Companion To Evangelical Theology, edited by Timothy Larsen and Daniel J. Treier, pp. 19-20
 A.M. Fairweather, ‘General Introduction’ to Aquinas on Nature and Grace: Selections from the Summa Theologica, pp. 21-22, 28