τὸ ὄν
Plato's philosophical term for God, meaning "that which is." Lapide, via Justin, argues this was borrowed from Moses, who calls God "ehyeh" ("who is," or "I am who I am").
Preface and Praise of Sacred Scripture
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Praises of Moses from Scripture and the Fathers
— Lapide explains that Plato called God "to on" because he learned the concept from Moses, who calls God "ehyeh"
"Plato learned about God from Moses, whence he likewise called Him 'to on,' that is, 'that which is,' just as Moses calls Him 'ehyeh,' that is, 'who is,' or 'I am who I am.'"