In the seventeenth century, the Socinians were known as chief heretics. Their denial of the deity of Christ and, thus, the Trinity, of the sovereignty and foreknowledge of God, of original sin, and of hell, among other orthodox teachings, meant that they faced strong opposition from the Reformed, Lutherans, and Roman Catholics alike. A fair bit of scholarship over the last couple of centuries has been dedicated to Socinian Christology and Soteriology, as well as to the denial of the doctrine of Trinity. But the opposition offered by seventeenth-century orthodox theologians often picked at a matter underlying the heretical Socinian positions. As Sarah Mortimer glosses it, according to Reformed theologians, the Socinians “were driven to their heresy by their inability to understand the biblical text, a fault which stemmed from their poor grasp of the fundamental terms of metaphysics.”  This poor grasp was not merely a failure to remember definitions of metaphysical terms. The point is that the Socinians, however well trained in the humanistic arts of the renaissance, had failed to understand metaphysics—or, better, Christian metaphysics—which had in its development in the West enabled Christian theologians to offer thick interpretations of Sacred Scripture.
An example of this kind of orthodox opposition to Socinianism comes to us from John Owen, the seventeenth-century English theologian. He says that many come to the doctrine of the trinity, for example, and begin to doubt because of terms such as “Trinity, essence, substance, persons, personality, respects, properties,” and the like. Where are these found in Scripture, they ask. And isn’t this overly speculative? Or worse, isn’t this evidence that Greek philosophical categories have quite taken over? These are not bad questions, Owen makes clear. But they need a clear answer lest they progress from wonderings to outright denial (which is, he says, what has happened with Socinians). The answer is that these terms and their modes of expression are explanations of the biblical revelation. What is that biblical revelation? In sum: “that God is one;— that this one God is Father, Son and Holy Ghost;— that the Father is the Father of the Son; and the Son, the Son of the Father; and the Holy Ghost, the Spirit of the Father and the Son; and that, in respect of this their mutual relation, they are distinct from each other.” 
That is the Scriptural testimony concerning God, which we as creatures are summoned to believe, on the authority of God himself. But it is difficult to understand. And this is precisely where the explanations come in. These explanations do not make understanding easy, they make it possible. So, with much care and a good deal of effort, one takes the sublime revelation of Scripture that God is one and Father, Son, and Spirit, and prayerfully works through the explanations. What happens when, on account of the difficulty, one turns back and eschews the explanations, worked out as they are metaphysically? One’s vision of Scripture itself can become blurred, and then the results can be disastrous.
In his most thorough response to the Socinians, his Vindiciae Evangelicae (1655), Owen concludes with a catechism that he pulled from the teachings of a Socinian, John Biddle, on the doctrine of God. It is a satirical piece, but Owen thought it fairly represents the results of the Socinian reductionistic reading of Scripture:
Ques. 1. What is God?
Ans. God is a spirit, that hath a bodily shape, eyes, ears, hands, feet, like to us.
Q. 2. Where is this God?
A. In a certain place in heaven, upon a throne, where a man may see from his right hand to his left.
Q. 3. Doth he ever move out of that place?
A. I cannot tell what he doth ordinarily, but he hath formerly come down sometimes upon the earth.
Q. 4. What doth he do there in that place?
A. Among other things, he conjectures at what men will do here below.
Q. 5. Doth he, then, not know what we do?
A. He doth know what we have done, but not what we will do. 
This may seem not only satirical but overly hyperbolic as well. Surely Owen has misunderstood them. But history tells a different story. As Sascha Salatowsky has shown, the influence of Socinian philosophy and theology on some of the more radical thinking of the Enlightenment, has been until recently vastly overlooked. Socinianism played a role on par with the likes of the thought of Spinoza, Hobbes, and Locke.  “Their radical rejection of the core Christian tenets, as the trinity, incarnation, justification, original sin and so forth, their attempt to create a rational religion was one of the crucial starting points for the early Enlightenment…”  In particular, Socinian thought became an important intellectual link in the chain of the development from pre-Modern forms of philosophical dualism to modern philosophical materialism. 
Put in broad terms, the development of Socinian thought contributed to the reduction of metaphysics, which treats of the supra-natural order—what is beyond that which is immediately available to the sense—to physics, which treats of the natural order. Is it any wonder, then, that in Socinian theology, God resembles a creature much more than the Creator?
This was not only the opinion of Owen and his fellow Reformed Orthodox. The philosopher Gottfried Wilhelm Leibniz likewise stated:
“Everyone knows that those who follow the theology of Faustus Socinus and his kind, and assail many mysteries of the Christian faith, but chiefly the mysteries of the divine Trinity in unity and the incarnation of the divine nature in Christ, have spawned their own philosophy; a philosophy which stands opposed to what was used—and for the most part still is used—and esteemed not only in Roman Catholic universities, but also frequently so in Protestant universities as well.” 
An examination of Socinian theology and philosophy, Leibniz continues, reveals
the account that otherwise learned and sharp men, abusing by their own cleverness not only theology but also a certain kind of shriveled philosophy, give to us, in which account hardly anything majestic or sublime remains, but God himself is pretty well relegated to the creaturely order. And this on account of our minds also being turned back and consigned to the material realm only. 
We might put the matter this way: much of the Socinian denials of the orthodox understanding of the person and work of Christ and of the doctrine of the Trinity rested on a reductionistic metaphysics.
This is not to say that the real, or only, problems of the Socinians were philosophical problems. It is not to say that philosophy is more important than theology, or that if one gets his metaphysics right, then his theology will fall in properly. There were, of course, a great many exegetical problems in Socinian theological positions. There were also numerous logical problems. But that is the point: in any interpretation of Scripture, in its explication and explanation, theology and philosophy do, for better or worse, walk hand in hand. The attempts to excise theology of metaphysics eventuates in a shriveled philosophy that in turn withers theology.
The solution to such withered theology is not, of course, a hypertrophied metaphysics. We must take care not to swing the pendulum to the other extreme. Rather, the solution is a properly Christian metaphysics; a proper synthesis of reason and faith. If we are at least brought so far as to that conclusion, then we are in the right way to begin the difficult but immensely privileging task of uttering creaturely speech about this One who is no creature. Or, to put it in Leibniz’s terms, to attempt an account of the Most Majestic and Sublime. For this, metaphysics is a most useful ancillary. For this, the enlightening Spirit of God is utterly necessary.
  Owen, Works, 12:588.
  Sascha Salatowsky, “Socinian Headaches: Adriaan Koerbagh and the Antitrinitarians,” in
  Ibid, 167.
  See, for example, Falk Wunderlich. “Materialism in Late Enlightenment Germany: A Neglected Tradition Reconsidered.” British Journal for the History of Philosophy 24, no. 5 (September 2, 2016): 940–62; and the fuller studies of Kestutis Daugirda, Die Anfänge des Sozinianismus  and Sascha Salatowsky, Die Philosophie der Sozinianer .
  From Leibniz, Ad Christophori Stegmanni Metaphysicam Unitariorum, in Nicholas Jolley, “An Unpublished Leibniz MS on Metaphysics”, Studia Leibnitiana, 7 (1975): 176–189; cited 176. Translation my own.
  Ibid, 177.