Augustine of Hippo — as excerpted in the Catena Aurea on John 6:1-14

“(lib. lxxxiii. Quæst. q. 61. in princ.) The five barley loaves signify the old law; either because the law was given to men not as yet spiritual, but carnal, i. e. under the dominion of the five senses, (the multitude itself consisted of five thousand:) or because the Law itself was given by Moses in five books. And the loaves being of barley is also an allusion to the Law, which concealed the soul’s vital nourishment, under carnal ceremonies. For in barley the corn itself is buried under the most tenacious husk. Or, it alludes to the people who were not yet freed from the husk of carnal appetite, which cling to their heart.”