Augustine of Hippo — as excerpted in the Catena Aurea on John 6:15-21

“(Tr. xxv. s. 3. et seq.) There is a mystical meaning in our Lord’s feeding the multitude, and ascending the mountain: for thus was it prophesied of Him, So shall the congregation of the people come about Thee: for their sake therefore lift up Thyself again: (Ps. 7) i. e. that the congregation of the people may come about Thee, lift up Thyself again. But why is it fled; for they could not have detained Him against His will? This fleeing has a meaning; viz. that His flight is above our comprehension; just as, when you do not understand a thing, you say, It escapes me. He fled alone unto the mountain, because He is ascended from above all heavens. But on His ascension aloft a storm came upon the disciples in the ship, i. e. the Church, and it became dark, the light, i. e. Jesus, having gone. As the end of the world draws nigh, error increases, iniquity abounds. Light again is love, according to John, He that hateth his brother is in darkness. (1 John 2:9) The waves and storms and winds then that agitate the ship, are the clamours of the evil speaking, and love waxing cold. Howbeit the wind, and storm, and waves, and darkness were not able to stop, and sink the vessel; For he that endureth to the end, the same shall be saved. (Matt. 10:22) As the number five has reference to the Law, the books of Moses being five, the number five and twenty, being made up of five pieces, has the same meaning. And this law was imperfect, before the Gospel came. Now the number of perfection is six, so therefore five is multiplied by six, which makes thirty: i. e. the law is fulfilled by the Gospel. To those then who fulfil the law Jesus comes treading on the waves, i. e. trampling under foot all the swellings of the world, all the loftiness of men: and yet such tribulations remain, that even they who believe on Jesus, fear lest they should be lost.”