Platonism
In the philosophy of Plato, the beatific vision is the vision of the Good. In Plato's Allegory of the cave, which appears in the Republic Book 7 (514a – 520a), he writes (speaking, as he does in many of his works, through the character of Socrates):
"My opinion is that in the world of knowledge the idea of good (the Good) appears last of all, and is seen only with an effort; and, when seen, is also inferred to be the universal author of all things beautiful and right, parent of light and of the lord of light in this visible world, and the immediate source of reason and truth in the intellectual." (517b,c)
Thus, for Plato, the Good appears to correspond to God in Christian theology.
St. Augustine expressed views similar to Plato's on this subject, and was familiar with Plato's ideas, most likely via Neoplatonist writings.
In the Eleusinian Mysteries
Socrates' mystic vision of initiation from Plato's Phaedrus.
There was a time when with the rest of the happy band they saw beauty shining in brightness, – we philosophers following in the train of Zeus, others in company with other gods; and then we beheld the beatific vision and were initiated into a mystery which may be truly called most blessed, celebrated by us in our state of innocence before we had any experience of evils to come, when we were admitted to the sight of apparitions innocent and simple and calm and happy, which we beheld shining in pure light. Phaedrus:250
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