Plantinga, Thompson, and Lundberg (PTL henceforth) make the following proposal:
Along with other notable theologians, we contend that much of modern atheism is a reaction to a concept of God that is more influenced by classical theism than a trinitarian theism, and that a robust trinitarianism, in addition to being a more biblical kind of theism, can serve as an important apologetic response to this lingering crisis of faith in the late-modern western world (An Introduction to Christian Theology, p. 91).
What therefore is the difference between classical theism and trinitarian theism? In summarizing what they suppose is an attempted reconciliation of competing worldviews on the part of the patristic authors -- viz., that of Greek philosophical theism, on the one hand, and of the trinitarian biblical narrative, on the other -- they effectively make the distinction along metaphysical and personalist lines. Classical theism is a heavily philosophical and metaphysical conception of God, whereas the other is described in large time-indexed and personal terms.
One finds in the church fathers, therefore, a fairly standard treatment of divine attributes. On the one hand, there are trinitarian affirmations as required by the biblical text -- for instance, that God in Christ, the eternal Son of the Father, became incarnate, suffered, died, was bodily resurrected, and will return again in judgment, all of which on the face of it seems to imply a divisibility, mutability, passibility, pathos, and temporality in God But on the other hand, one also finds an array of Greek metaphysical attributes that assert the opposite -- for example, that God is indivisibly simple, absolutely immutable (and therefore impassible or apathetic), and timelessly eternal, a metaphysic that the church fathers largely regarded as consistent with the biblical terminology that God is One (Deut. 6:4), unchanging (e.g., Mal. 3:6), and eternal or everlasting (e.g., Ps. 90:2) (p. 87).
Why would anyone be a classical theist? Why would anyone suppose God to have those attributes? PTL record some of the Middle Platonist-style arguments one might offer:
Middle Platonism tended to view God as an impersonal principle of supreme being -- "God" as the immaterial and transcendent ground, unity, and structure of being, the veritable fullness of life. This theistic concept evolved mainly out of the early philosophical question of "the One and the many." Its basic theo-logic runs as follows: given that our experience in the material world is of the manifold, of the "many" -- things, aspects, moments, and so on -- there must be a "One" that transcends, grounds, and unifies the many-ness of life and gives it coherence and meaning. For the early Greek philosophers, material multiplicity was a problem in itself, since everything with parts eventually over time changes for the worse; everything with parts wears down, deteriorates, dies, and, by all appearances, dissolves into oblivion -- witness, for example, the human body. Only an immaterial transcendent One, without parts, would be immune to this physics of decay and death. This One Supreme Being, or God, would be characterized by the highest perfection, and so would be the supreme instance of all the positive or good things in life: the all-true, good, beautiful, happy, self-sufficient One. Conversely, God would be the opposite of all the bad things. Therefore many of the divine attributes must be expressed as negations. Since material parts or divisibility lead to decay and death, God would have to be immaterial and indivisible -- One simple, single spiritual thing. As the transcendent One without parts, God would be free from change and therefore immutable, since change involves being affected by another (moreover, if God could change, this would entail that God is not perfect -- since change is assumed to be either for the worse or the better, and God must by definition already be the best). As immutable, God must also be impassible or apathetic -- neither affected by another, not even emotionally, nor prone to suffering. All of this entails that the One could not be temporal or "ternal" -- that is, in time, the realm of transience and death -- but rather eternal, dwelling in timeless transcendence (pp. 85-6).
Now PTL have affirmed that various forms of modern atheism are a response to the kind of conception of God drawn from Middle and Neoplatonist philosophy. What becomes obvious as you read their treatments of the varieties of atheism, however, is that none of them actually address anything resembling classical theism. Therefore PTL's claim that modern atheism arose as a response to classical theism is fatally underdetermined by the evidence they bring forth.
Consider what they call scientific atheism: As modern science developed and the world was being discovered in its natural integrity as a well-oiled machine run by natural laws (the Newtonian world), less and less was God needed as an explanatory principle of the world's workings. Life phenomena that were mysterious to the ancients and medievals -- take thunderstorms or earthquakes -- were generally attributed to the spiritual forces of the gods/God. As the operations of the world were discerned as natural laws, the cosmos was significantly demystified and God was increasingly pushed to the periphery of human knowledge (p. 93). This is the atheism of Pierre Laplace, who told Napoleon that he had no need of the God hypothesis to explain the structure of the physical universe.
That these persons are not responding to classical theism is obvious to those with a mind attuned to subtle logical distinctions -- or not so subtle, as in the present case. PTL themselves, when recounting in admittedly abbreviated and inadequate form the arguments of the Middle Platonists to the One, made no reference to natural phenomena of mysterious origins. It was metaphysical facts -- multiplicity, the existence of composites, the nature of change, etc. -- that motivated their arguments, not natural phenomena subject to empirical investigation. When Plotinus argues in Enneads VI.9 that composite objects have their existence from the unity of their ontological constituents, and therefore there must be a simple metaphysical Absolute which gives existence of itself to everything else, he was not offering an argument from mysterious natural phenomena subject to empirical investigation by Enlightenment-period science. He was offering an argument on metaphysical premises that cannot be empirically investigated or denied. So scientific atheism was not a response to classical theism; it didn't even touch on the same plane of reality from which classical theism drew its arguments and evidences, and which it addressed.
They also speak of humanistic atheism, the sort of atheism that sees belief in God as an impediment to human progress and self-actualization. They cite Feuerbach: Put most simply, Feuerbach thought that the notion of God was a projection of human imagination; "God" was a human fancy, a person blown up to infinite, eternal size . . . Moreover, Feuerbach considered theism a pathology: humanity projects an image o itself as "God" and lavishes on it all our best features and attributes in God-sized measure, but then so objectifies this image transcendentally that in view of God humans become but poor, wretched sinners, unable to do any good. . . . For Feuerbach, all theology was really anthropology, and to face up to this truth is the first step in liberating humanity for positive social and political action in the world (p. 94). Humans anthropomorphize a God far beyond what they are themselves, find themselves small and insignificant before the reification of their own ideals, and are paralyzed
I don't see any projection in the Middle Platonist arguments for the existence of the One; I don't see any projection in Plotinus' arguments, but rather insistent rejection of any anthropomorphism of the divine and persistence in upholding the utter transcendence of the One beyond all human concepts. PTL give the impression that the classical theistic conception of God is irreligiously impersonal and abstract compared to the biblical one (p. 100), and then propose that Feuerbach's proposal of anthropomorphism as a response to classical theism. This makes no sense. Granting gladly that human beings are prone to anthropomorphize God and understand him as a kind of cosmic, massive version of ourselves, none of the Middle Platonist or classical theistic arguments in general are thereby refuted by pointing out this fact. In fact all of this is a gigantic non sequitur.
Then again there is apathetic atheism, the kind of atheism that is motivated by a lack of concern for God. Happy with the content of the life he lives, the apathetic atheist doesn't think about God and doesn't care to investigate the matter of his existence. There is obviously no reaction here, since the apathetic atheist doesn't even engage in intellectual investigation of matters of philosophy and religion anyway.
Finally there is protest atheism, the kind of atheism that is a passionate remonstrance against God in view of the fractured human condition. That is, it takes up the plight of humanity in the face of seeming divine indifference (p. 96). They describe a couple of types of protest atheism.
On the one hand, there is Albert Camus's protest atheism grounded in the absurdity of life: . . . in the absence of metaphysical absolutes or any pattern of life's meaning, as traditionally grounded in God, the individual must resolutely define and "authenticate" his or her own life. . . . For Camus, the absurdity of life appears incompatible with any notion of providential governance of the world; rather, the world seems to be inhabited by "a god who had come into it with a dissatisfaction and a preference for futile sufferings." . . . Like others, Camus cannot reconcile traditional theism with the absurd conditions of human existence (p. 98).
It is hard to see how this is a response aimed distinctly at classical theism. This is a problem, if it is a problem, with any conception of God which admits of a providence of God. Appealing to Ivan Karamazov's remarks in The Brothers Karamazov, they summarize protest atheism: the problem of human suffering renders any notion of divine providence questionable (p. 99). If God knew what the world would be like before he made it, if he knew what he was getting into (so to speak) when he decided to create, then you have the problem that the world as it appears to us seems less than what we would expect of him. We might also make the complaint that protest atheism such as this is not a reaction to classical theism since it makes no effort to refute any of its arguments. Even if God did not care one bit for the world, that would not refute the arguments for his existence that the classical theistic tradition offers.
Summary and conclusion: the forms of modern atheism that PTL recount are not genuine reactions to classical theism. They do not touch upon or ever refute the classical theistic arguments, and they do not even talk about the same things. Even granting the protest atheist's point that the evils of terrestrial existence make the existence of divine providence unlikely, an admission that we do not have to make, we are not thereby refuted as classical theists. The arguments for the existence of the One still stand. In this way we see that modern atheism is largely unmotivated and groundless from the perspective of classical theism.
There is more to be said, of course.