Luke 5:4

We have been toiling all night. This refers symbolically to the prophets. His teaching came down from on high on the world, which stands by way of parable for the sea. The two boats represent the circumcised and the uncircumcised. They made a sign to their companions. This refers symbolically to the seventy-two, for these disciples were too few in number for the catch and the harvest.

Now in a mystery, the ship of Peter, according to Matthew, is beaten about by the waves, (Matt. 8:24.) according to Luke, is filled with fishes, in order that you might understand the Church at first wavering, at last abounding. The ship is not shaken which holds Peter; that is which holds Judas. In each was Peter; but he who trusts in his own merits is disquieted by another's. Let us beware then of a traitor, lest through one we should many of us be tossed about. Trouble is found there where faith is weak, safety here where love is perfect. Lastly, though to others it is commanded, Let down your nets, to Peter alone it is said, Launch out into the deep, i. e. into deep researches. What is so deep, as the knowledge of the Son of God! But what are the nets of the Apostles which are ordered to be let down, but the interweaving of words and certain folds, as it were, of speech, and intricacies of argument, which never let those escape whom they have once caught. And rightly are nets the Apostolical instruments for fishing, which kill not the fish that are caught, but keep them safe, and bring up those that are tossing about in the waves from the depths below to the regions above. But he says, Master, we have toiled the whole night and have caught nothing; for this is not the work of human eloquence but the gift of divine calling. But they who had before caught nothing, at the word of the Lord inclosed a great multitude of fishes.

(Hom. 6. in Matt.) For in His condescension to men, He called the wise men by a star, the fishermen by their art of fishing.

He told Simon and his companions to sail off a little from the land and to let down the net for a draught. But they replied that they had been toiling the whole night and had caught nothing. However, in the name of Christ, they let down the net, and immediately it was full of fish. By a visible sign and by a miraculous type and representation, they were fully convinced that their labor would be rewarded, and the zeal displayed in spreading out the net of the gospel teaching would be fruitful. Within this net they should most certainly catch the shoals of the heathen. But note that neither Simon nor his companions could draw the net to land. Speechless from fright and astonishment—for their wonder had made them mute—they beckoned to their partners, to those who shared their labors in fishing, to come and help them in securing their prey. For many have taken part with the holy apostles in their labors, and still do so, especially those who inquire into the meaning of what is written in the holy Gospels. Yet besides them there are also others: the pastors and teachers and rulers of the people, who are skilled in the doctrines of truth. For the net is still being drawn, while Christ fills it, and calls to conversion those who, according to the Scripture phrase, are in the depths of the sea, that is to say, those who live in the surge and waves of worldly things.

Having sufficiently taught the people, He returns again to His mighty works, and by the employment of fishing fishes for His disciples. Hence it follows, When he had left off speaking, he said unto Simon, Launch out into the deep, and let down your nets for a draught.

He chooses Peter’s boat and forsakes Moses’—that is to say, he spurns the faithless synagogue and takes the faithful church. For God appointed the two as boats, so to speak, which would fish for the salvation of humankind in this world as in a sea. As the Lord says to the apostles, “Follow me, and I will make you fishers of men.” …The church is called out into the deep, delving, as it were, into the profound mysteries of the heavens, into that depth concerning which the apostle says, “O the depth of the riches and wisdom and knowledge of God!” For this reason he says to Peter, “Put out into the deep,”—that is to say, into the depths of reflection upon the divine generation. For what is more profound than what Peter says to the Lord, “You are the Christ, the Son of the living God?” …
This boat sails upon the deeps of this world, so that, when the earth is destroyed, it will preserve unharmed all those it has taken in. Its foreshadowing can be seen already in the Old Testament. For as Noah’s ark preserved alive everyone whom it had taken in when the world was going under, so also Peter’s church will bring back unhurt everyone whom it embraces when the world goes up in flames. And as a dove brought the sign of peace to Noah’s ark when the flood was over, so also Christ will bring the joy of peace to Peter’s church when the judgment is over.

Twice in the holy Gospel it is read that the Lord commanded that nets be cast for fishing, namely before the Passion and after the Resurrection. But before our Redeemer suffered and rose again, He commands the net to be cast for fishing, but does not command whether it should be cast on the right or on the left; however, appearing to the disciples after the Resurrection, He commands the net to be cast on the right. In that fishing so many were caught that the nets were torn; but in this one both many were caught and the nets were not torn. Who indeed does not know that the good are signified by the right and the wicked by the left? That fishing, therefore, in which it is not specifically commanded on which side the net should be cast, designates the present Church, which gathers the good together with the wicked, and does not choose whom it draws in, because it does not know whom it might choose. But this fishing done after the Lord's Resurrection was cast only on the right, because only the Church of the elect attains to seeing the glory of His brightness, which will have nothing from sinful works. In that fishing the net is torn because of the multitude of fish, because now so many reprobate enter into the confession of faith along with the elect that they even tear the Church itself apart with heresies. But in this fishing both many fish and large ones are caught, and the net is not torn, because the holy Church of the elect, resting in the continual peace of its Author, is no longer torn apart by any dissensions.

But when he had ceased speaking, he said to Simon: Put out into the deep and let down your nets for a catch. That he first asked Simon to put out the boat a little from the land signifies either to use the word cautiously with the crowds, so that neither earthly things are prescribed to them nor they are removed so deeply from earthly things into the depths of the sacraments that they do not understand them at all, or first to preach to the nearby regions and peoples, so that when he later says again to Peter, "Put out into the deep and let down your nets for a catch," it pertains to the more distant nations to whom the preaching was later extended.