9 It was not, then, without a purpose that these fishes were described as so many in number, and so large in size, that is, as both an hundred and fifty-three, and large. For so it is written, “And He drew the net to land full of great fishes, an hundred and fifty and three.” For when the Lord said, “I am not come to destroy the law, but to fulfill,” because about to give the Spirit, through whom the law might be fulfilled, and to add thereby, as it were, seven to ten; after interposing a few other words He proceeded, Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
The latter, therefore, may possibly belong to the number of great fishes. But he that is the least, who undoes in deed what he teaches in word, may be in such a church as is signified by that first capture of fishes, which contains both good and bad, for it also is called the kingdom of heaven, as He says, “The kingdom of heaven is like a net that was cast into the sea, and gathered of ever kind;” where He wishes the good as well as the bad to be understood, and of whom He declares that they are yet to be separated on the shore, to wit, at the end of the world.
And lastly, to show that those least ones are reprobates who teach by word of mouth the good which they undo by their evil lives, and that they will not be even the least, as it were, in the life that is eternal, but will have no place there at all; after saying, “He shall be called the least in the kingdom of heaven,” He immediately added, “For I say unto you, That unless your righteousness shall exceed [the righteousness] of the scribes and Pharisees, you shall not enter into the kingdom of heaven.” Such, doubtless— these scribes and Pharisees— are those who sit in Moses' seat, and of whom He says, “Do ye what they say, but do not what they do; for they say, and do not.” They teach in sermons what they undo by their morals.
It therefore follows that he who is least in the kingdom of heaven, as the Church now exists, shall not enter into the kingdom of heaven, as the Church shall be hereafter; for by teaching what he himself is in the habit of breaking, he can have no place in the company of those who do what they teach, and therefore will not be in the number of great fishes, seeing it is he “who shall do and teach that shall be called great in the kingdom of heaven.” And because he will be great here, therefore shall he be there, where he that is least shall not be.
Yea, so great will they certainly be there, that he who is less there is greater than the greatest here. And yet those who are great here, that is, who do the good that they teach in that kingdom of heaven into which the net gathers good and bad, shall be greater still in that eternal state of the heavenly kingdom,— those, I mean, who are indicated by the fishes here as belonging to the right hand and to the resurrection of life. We have still to discourse, as God shall grant us ability, on the meal that the Lord took with those seven disciples, and on the words He spoke after the meal, as well as on the close of the Gospel itself; but these are topics that cannot be included in the present lecture.
Source: Tractates on the Gospel of John (New Advent)