1 It was a great miracle that was wrought, dearly beloved, for five thousand men to be filled with five loaves and two fishes, and the remnants of the fragments to fill twelve baskets. A great miracle: but we shall not wonder much at what was done, if we give heed to Him That did it. He multiplied the five loaves in the hands of them that broke them, who multiplies the seeds that grow in the earth, so as that a few grains are sown, and whole barns are filled. But, because he does this every year, no one marvels.
Not the inconsiderableness of what is done, but its constancy takes away admiration of it. But when the Lord did these things, He spoke to them that had understanding, not by words only, but even by the miracles themselves. The five loaves signified the five books of Moses' Law. The old Law is barley compared to the Gospel wheat. In those books are great mysteries concerning Christ contained. Whence He says Himself, “If you had believed Moses, you would believe Me also; for he wrote of Me.” But as in barley the marrow is hid under the chaff, so in the veil of the mysteries of the Law is Christ hidden.
As those mysteries of the Law are developed and unfolded; so too those loaves increased when they were broken. And in this that I have explained to you, I have broken bread unto you. The five thousand men signify the people ordered under the five books of the Law. The twelve baskets are the twelve Apostles, who themselves too were filled with the fragments of the Law. The two fishes are either the two precepts of the love of God and our neighbour, or the two people of the circumcision and uncircumcision, or those two sacred personages of the king and the priest. As these things are explained, they are broken; when they are understood, they are eaten.
Source: Sermons on Selected Lessons of the New Testament (New Advent)