Wayne Hankey - Iamblichean/Dionysian principles changed in Aquinas' ontology

Equally, Denys, and the Neoplatonism which derives from Iamblichus, are responsible for the principles governing celestial hierarchy in Aquinas, for the enormous role it plays in his system, for the distinctions of its forms and ranks and for their names. However, a great part of the purpose of this hierarchy for Denys is lost when it is moved into Thomas’ world. What intervenes is Thomistic ontology. He insists rigorously on a distinction between the donation of substantial being, which is the creative act, and all other subsequent donations. The higher angels confer knowledge but not, as in Denys, being, grace, and glory on the lower spirits. Moreover, the equality of humans as compared to angels prevents hierarchical communication in the celestial order being the model for that communication in the church. This modeling is fundamental to Denys’ purpose.
"Dionysian Hierarchy in St. Thomas Aquinas: Tradition and Transformation,"

The compromise does not bring out, indeed, it conceals, the principle which determines the matter for Denys and which will determine it again for Aquinas; i.e. hierarchical action is always mediated. The highest of one order never touches a lower order. There must be a diminution of spiritual virtue to a lower level within a hierarchical rank before a higher order of being can come into contact with a lower. Even then the higher touches only the top grade of the lower order. This characteristically Iamblichan and Procline principle, well known to Aquinas, and accepted by him
...Thomas’ identification of the Iamblichan law of hierarchical mediation with the law of grace
"Aquinas, Pseudo-Denys, Proclus and Isaiah VI.6,"

A consequence of Thomas’ submission to the authority of Denys is that he is essential to constructing Latin intellectual and political systems. Doubtless, Denys is transmuted in the systems of intellectual and institutional power he enables. But ironically, this is precisely because the logic he conveys is more inclusively dialectical than those Aristotle or Augustine provided Aquinas. Though he represents the Greek theological tradition, but carries from it a logic of such synthetic power, because of its embedded Iamblichan - Procline Neoplatonism, Denys has been essential to making Latin Christendom the most potent tradition. Whether that power is only the power for the greatest of self-overcomings or has another fate is also profoundly a question of what the Dionysian corpus really is about. Above all it is a question of the difference between what Denys said and what Aquinas heard, together with the questions as to whether we are either more profound, or more accurate, listeners than Thomas was. The trouble is that we shall be able to answer none of these pressing and difficult questions about the discontinuities, if we do not recognize the continuities.
"Denys and Aquinas: Antimodern Cold and Postmodern Hot,"

Thomas’ hermeneutical horizon was profoundly, extensively and subtly Platonist.
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Thomas wrote a commentary on his De Divinis Nominibus which is our most important source for his knowledge and judgment of Platonism until the works which follow his reading of Proclus’ Elements of Theology. But, as we shall see below, the pervasive Platonism of the Dionysian corpus was hidden to Aquinas for some time. Because of purposes which remain unknown to us, Dionysius not only hid his identity but also obscured his relation to Platonism. In consequence, Thomas had little control over how the Neoplatonism Dionysius authoritatively conveyed affected him.
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in the passage from the very late De Substantiis Separatis quoted just above, the doctrine that God is the solitary cause of being for all things is stated in a form which sounds more Platonic than Aristotelian. Thomas speaks there of the First Principle as simplicissimum and argues that “because subsistent being (esse subsistens) must be one ... it is necessary that all other things which are under it exist in the way they do as participants in esse (omnia alia quae sub ipso sunt sic esse quasi esse participantia).”[46] His Exposition of the Liber de causis shows that, having looked at Plato more and more in Neoplatonic terms, Thomas saw that for Platonists all is derived from one exalted First Principle. Even if the Platonists “posited many gods ordered under one” rather than as we do “positing one only having all things in itself”, everyone agrees “universality of causality belongs to God” (universalitas .. causalitatas propia est Deo).
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Thomas’ assessment of Platonism takes place from within it.
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Plotinus, Simplicius, Proclus, the Arabic Neoplatonic Aristotelians are not Aquinas’ only sources for this idea crucial to reconciling Plato and Aristotle. Thomas also finds it in the Pseudo-Dionysius. [same sources for Pico's reconciliation of Plato and Aristotle]
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Thomas’ criticism of Plato, a criticism as much Neoplatonic as it is Aristotelian.
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Thomas’ view of Aristotelianism had been shaped within the tradition of Neoplatonic commentary, aspects of which his own work continued, and partly as a result of the traditional ascription of the Liber to Aristotle. This, at least to a degree, accounts for his thinking that “Dionysius nearly everywhere follows Aristotle”. After reading the Elements of Theology and comparing it with the Liber and the writings of Dionysius, some of his earlier judgments about the Aristotelianism of the two monotheistic authors were confirmed.
Making this comparison, Thomas found that the doctrine of the De Divinis Nominibus was a monotheistically modified Platonism like that of the Liber de causis. The author of the Liber reduces the plurality of the divine hypostases. With the consequent elevation of the First, its creativity as absolute source is correspondingly secured and exalted. Looking at the doctrine of Proclus, his view is confirmed that even the Platonists teach that the First is the cause of the substance of what follows it. Nonetheless, he judges that the doctrine of creation is more securely taught by Aristotle because of his refusal to posit universals existing per se. In the exposition of the Liber, Aquinas associates its author and Dionysius with Aristotle in a criticism of features of pagan Platonism. He uses Dionysius to correct them all.
“Aquinas and the Platonists,”