5 To run over it briefly: by the five loaves are understood the five books of Moses; and rightly are they not wheaten but barley loaves, because they belong to the Old Testament. And you know that barley is so formed that we get at its pith with difficulty; for the pith is covered in a coating of husk, and the husk itself tenacious and closely adhering, so as to be stripped off with labor. Such is the letter of the Old Testament, invested in a covering of carnal sacraments: but yet, if we get at its pith, it feeds and satisfies us.
A certain lad, then, brought five loaves and two fishes. If we inquire who this lad was, perhaps it was the people Israel, which, in a childish sense, carried, not ate. For the things which they carried were a burden while shut up, but when opened afforded nourishment. And as for the two fishes, they appear to us to signify those two sublime persons, in the Old Testament, of priest and of ruler, who were anointed for the sanctifying and governing of the people. And at length Himself in the mystery came, who was signified by those persons: He at length came who was pointed out by the pith of the barley, but concealed by its husk.
He came, sustaining in His one person the two characters of priest and ruler: of priest by offering Himself to God as a victim for us; of ruler, because by Him we are governed. And the things that were carried closed are now opened up. Thanks be to Him. He has fulfilled by Himself what was promised in the Old Testament. And He bade the loaves to be broken; in the breaking they are multiplied. Nothing is more true. For when those five books of Moses are expounded, how many books have they made by being broken up, as it were; that is, by being opened and laid out?
But because in that barley the ignorance of the first people was veiled, of whom it is said, “Whilst Moses is read, the veil is upon their hearts;” for the veil was not yet removed, because Christ had not yet come; not yet was the veil of the temple rent, while Christ is hanging on the cross: because, I say, the ignorance of the people was in the law, therefore that proving by the Lord made the ignorance of the disciple manifest.
Source: Tractates on the Gospel of John (New Advent)