Augustine of Hippo — as excerpted in the Catena Aurea on Luke 11:9-13

“(de Quæst. Ev. lib. ii. qu. 22.) Or by the bread is meant charity, because we have a greater desire of it, and it is so necessary, that without it all other things are nothing, as the table without bread is mean. Opposed to which is hardness of heart, which he compared to a stone. But by the fish is signified the belief in invisible things, either from the waters of baptism, or because it is taken out of invisible places which the eye cannot reach. Because also faith, though tossed about by the waves of this world, is not destroyed, it is rightly compared to a fish, in opposition to which he has placed the serpent on account of the poison of deceit, which by evil persuasion had its first seed in the first man. Or, by the egg is understood hope. For the egg is the young not yet formed, but hoped for through cherishing, opposed to which he has placed the scorpion, whose poisoned sting is to be dreaded behind; as the contrary to hope is to look back, since the hope of the future reaches forward to those things which are before.”