It's quite strange to mention Epicurus in the same sentence as the ontological argument. Epicurus, after all, was among one of the most skeptical philosophers in ancient Greece. He denied the reality of divine providence insofar as it related to human affairs and he is famous for postulating one version of the argument from suffering.
Nevertheless, we surprisingly find Epicurus affirming the existence of the gods, who he maintained were composed of the most subtle atoms and were undergoing constant change, much like a waterfall is constantly filled with new water. Long before St. Anselm, Epicurus actually defended an ontological argument similar to that of Descartes's.
Epicurus's epistemology was exclusively based on sense-perception. Our concept of a unicorn exists only because we have perceptions of real entities, such as horses and various animals with horns. A unicorn is therefore a combination of the two and imbued with magical properties. Accordingly, Epicurus maintains, like Descartes, that our conception of the divine must be based in reality. No conception can exist apart from perception.
Whether this is a good argument is beside the point. It's just fascinating to find traces of the ontological argument over a thousand years before Anselm penned his Proslogion.