This article examines Thomas Aquinas’s critique of Anselm of Canterbury’s ontological argument, focusing particularly on its modal aspects. While Aquinas’s traditional objections (the fallacy of begging the question and the non-self-evidence of God’s definition) have been thoroughly studied, their modal interpretation requires further analysis. The article demonstrates that Aquinas rejects Anselm’s argument because it conflates the ontological necessity of God’s existence with the epistemic possibility of denying it due to human cognitive limitations. Five key texts from Aquinas are analyzed, in which he systematically refutes Anselm, arguing that God’s existence is selfevident only in itself (secundum se) but not to human reason. In the modal version of this critique, emphasis is placed on the denial of transitivity in the relation between possible worlds: the impossibility of comprehending the divine essence in the earthly state prevents the transition to the necessity of knowing God’s existence. Aquinas insists on an epistemic barrier that renders the ontological argument untenable for human cognition. The conclusion notes that while Aquinas’s critique aligns with broader Christian skepticism regarding human knowledge of divine reality in earthly existence, it does not refute the very possibility of God’s necessary existence.