When Saint Augustine told the boy that what the boy wanted to do was impossible, the boy revealed that he was an angel. He told Saint Augustine that what he had said about filling the hole with the ocean being impossible was true, but that it was more possible than for a human to understand the Mysteries of God. The analogy is clear: God is so far beyond our capabilities of understanding that we could not possibly grasp His entirety. However, I also think that it is important to note that the boy did have a piece of the ocean in the hole that he dug. While God might be beyond our understanding, He also makes Himself available to us in ways that we can understand.