Classical theism claims God is 'simple' — identical to his own existence, goodness, power, and every other attribute, with no distinct parts or properties — but this makes God indistinguishable from an abstract property and renders meaningful talk of God's distinct attributes (that he is both loving and just, for instance) incoherent.

Why people find it persuasive

If God's goodness just is his power which just is his existence which just is his wisdom, with no real distinction between any of them, then saying 'God is loving and also just' seems to collapse into a single, undifferentiated, mysterious blob-property that we can't meaningfully unpack, analyze, or apply the normal logic of distinct predicates to at all.

Important distinctions

• • Real distinction between parts vs. a merely conceptual/formal distinction between attributes that are really identical but conceived under different aspects.
• • Simplicity as denying God has any properties at all vs. denying God's properties are separable, composed, or metaphysically distinct from his essence.
• • God being identical to a single, generic 'existence' (an abstract, impersonal property) vs. God's existence being identical to a supremely perfect, personal act of being.
• • The classical Thomistic doctrine as originally formulated within a broader Aristotelian metaphysics of act/potency vs. popular caricatures of the doctrine.

Christian response

• • Classical theists (Aquinas being the paradigm case) do not deny that we can meaningfully and truthfully distinguish God's love, justice, wisdom, and power in our God-talk; the claim is precisely that this real, meaningful, truth-tracking distinction exists at the level of our finite conception and language (a virtual or formal distinction), tracking genuinely different effects and ways God's single, infinitely perfect act of being is manifested and known, while denying that these attributes are really separate metaphysical components or parts that could, even in principle, be pulled apart or that compose God the way parts compose a machine — simplicity denies composition, not distinguishability in thought and predication.
• • The motivation for simplicity is not an arbitrary mystical flourish but follows from a rigorous argument about what it would take to be the ultimate, uncaused explanatory terminus of all contingent reality: if God were composed of really distinct parts (even metaphysical parts like 'existence' and 'essence,' or distinct properties that could in principle be combined differently), something would need to explain why exactly this particular combination of parts obtains rather than some other combination, which would either invite an infinite explanatory regress or require an external composer — pushing the need for explanation back a level rather than terminating it. A truly ultimate explanatory stopping point is most coherently conceived as one whose existence and nature are not itself in need of further composition or external unification.
• • The comparison to an 'abstract property' misses that classical theism explicitly identifies God's simple existence as an act (actus purus, pure actuality) rather than a static abstract universal — abstract properties like 'redness' or 'triangularity' are not agents, do not think, will, love, or act; God's simplicity is the simplicity of an utterly unified, supremely actual, personal, self-existent act of being, radically different in kind from the simplicity of an inert Platonic abstraction, precisely because divine simplicity is paired in classical theism with equally central claims about God's personal knowledge, will, and love, not offered as a competing, impersonal alternative to them.
• • Contemporary analytic philosophers of religion sympathetic to classical theism (Eleonore Stump, Brian Davies, James Dolezal, Edward Feser) have developed careful, technically rigorous responses to the 'modal collapse' worry (the objection that if God's will and existence are identical, and God exists necessarily, then whatever God wills would also be necessary, seemingly eliminating any contingency, including the contingency of creation) by distinguishing the necessity of the divine nature/act from the freely willed relational effects of that act ad extra (toward creation) — a technically detailed but coherent solution that treats creation's contingency as located in the created effect's relation to God, not requiring composition within God himself.
• • It's also worth noting divine simplicity is not universally held even among serious classical theists — some contemporary analytic theologians and philosophers (including some who hold a robust classical theism in most other respects) qualify or reject strong simplicity in favor of a more modest doctrine (e.g., some forms of theistic personalism hold God has really distinct properties, simply all necessarily co-instantiated) precisely because of these difficulties, showing this is a genuine live internal debate within serious philosophical theology, not a case where the doctrine's defenders are forced to bite obviously absurd bullets to save an indefensible position.

Christian counterresponse

Classical theists respond that this dilemma presupposes a background metaphysics (that any true, non-identical predication must correspond to really distinct truthmaking components within the subject) that Thomistic and other classical accounts of analogical predication explicitly reject — on their account, our concepts of justice and mercy, formed by abstraction from finite, composite instances of justice and mercy, genuinely and truly (though only analogically, not univocally) apply to God's single, supremely perfect act, without requiring that act to literally contain separable components corresponding one-to-one with our distinct concepts; whether this theory of analogical predication is itself fully successful, however, is exactly the kind of technical question that keeps this a live and sophisticated debate in philosophical theology rather than a settled matter either way.