Divine simplicity follows from the Thomistic doctrine of pure actuality and is affirmed in the Roman Catholic tradition by the Fourth Lateran Council of 1215 and reaffirmed in Vatican I (1870).
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"...one of the most fundamental tenets of the Latin theological tradition [is] the doctrine of divine simplicity. Given divine simplicity as the Latin tradition understands it, God is identical with His own eternity, as He is identical with all of His essential attributes. This means that, as Augustine remarks, “eternity is the very substance of God” [1] Plainly since eternity is the divine substance, it cannot be shared by creatures[2]
[1]Expositions of the Psalms, Homily 2 on Psalm 101, ch. 10 (PL 37 1311).
[2]It is true that Aquinas speaks of angels and the blessed as “participating” in eternity, but on close examination this turns out to be an intentional rather than an ontological relationship. Such creatures participate in eternity only inasmuch as they take on the divine essence as an intelligible species. Given the identity of the act of understanding with its object, this means that they are united to God, as Aquinas puts it, not “in the act of being, but only in the act of understanding.” (Summa Contra Gentiles III.54.9; cf. III.61.3.)
David Bradshaw, A Christian Approach to the Philosophy of Time, pp 2, 12
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"When they [Roman Catholics] speak of operations and energies as distinct from the essence, they are thinking of created effects of the divine essence. Their notion of God admits of nothing but an essential existence for divinity. What is not the essence itself does not belong to the divine being, is not God. Therefore the energies must be either identified with the essence or separated from it completely as actions which are external to it, i.e. as created effects having the essence as their cause." Vladimir Lossky, cited in Aristotle Papanikolaou, Being With God: Trinity, Apophaticism, and Divine Human Communion (University of Notre Dame Press, 2006), p. 26.
"To this argument, Lossky, following Palamas, poses the dilemma that if God’s energies are not uncreated, either the essence is communicated which results not in deification but absorption, or created existence participates in something that is less than divine. In either case, there is no real deification that consists of a real communion with the divine without an annihilation of the integrity of creation. Barlaam’s defense of divine simplicity starts “from a philosophical concept of essence, leads finally to conclusions which are in admissible for practical piety and contrary to the tradition of the Eastern Church.” (Papanikolaou, ibid)
"...It thus becomes clearer what Lossky means when he argues that in Thomism “a rationalistic doctrine of causality is introduced into the doctrine of grace.” The net result is that it reduces knowledge of God to human concepts. For the God who is transcendent and immanent in the creation, this violates God’s transcendence insofar as God becomes that which is necessary according to modes of human thought and logical discourse; it also leads to a God who is not immanent insofar as these modes of human thought and logical discourse are limited to created being and do not bridge the gap between the uncreated and the created. The intellectualization of theology, with the introduction of a “rationalistic doctrine of causality,” leads necessarily to notions of created grace, which is something less than God... The main problem with such an approach to theology is that it fails to establish as its first principle the realism of divine-human communion. It attempts to eliminate the paradoxical nature of the Incarnation when the very core of theology is this antinomy. If the very notion of divine-human communion, of the transcendent and immanent God, is antinomic, then theological discourse itself is grounded in the very being of God and must express, not eliminate, the antinomy. Hence, the necessity for both cataphatic and apophatic theologies to be held in a tension that is transcended in the being of God, and not simply for an apophatic corrective to cataphatic theology. The apophatic affirmation of the God who is beyond being is, ironically for Lossky, the only way to affirm that there can be real immanence in created being and, thus, the realism of divine-human communion. (ibid, p. 29f.).
Aristotle Papanikolaou