There is always an ambiguity in this term of 'mere existence,' ipsum esse, auto to einai. Either it means ens abstractissimum, the thinnest and shallowest of concepts, denoting the barest removal from nothingness: or it is ens plenissimum, being that includes (virtually at least) all other being, as the Platonic auto to kalon virtually includes all beauty. In this latter sense the term is predicable of God alone. In God 'mere existence' means pure actuality.
Of God and His Creatures: 1.25