6 But give heed to the fact that even the hirelings are needful. For many indeed in the Church are following after earthly profit, and yet preach Christ, and through them is heard the voice of Christ; and the sheep follow, not the hireling, but the Shepherd's voice speaking through the hireling. Hearken to the hirelings as pointed out by the Lord Himself: “The scribes,” He says, “and the Pharisees sit in Moses' seat: do what they say; but do not what they do.” What else said He but, Listen to the Shepherd's voice speaking through the hirelings?
For sitting in Moses' seat, they teach the law of God; therefore God teaches by them. But if they wish to teach their own things, hear them not, do them not. For certainly such seek their own, not the things which are Jesus Christ's; but no hireling has dared to say to Christ's people, Seek your own, not the things which are Jesus Christ's. For his own evil conduct he does not preach from the seat of Christ: he does injury by the evil that he does, not by the good that he says.
Pluck the grapes, beware of the thorn. It is well I see that you have understood; but for the sake of those that are slower, I shall repeat these words with greater plainness. How said I, Pluck the bunch of grapes, beware of the thorn; when the Lord says, “Do men gather grapes of thorns, or figs of thistles”? That is quite true: and yet what I said is also true, Pluck the bunch of grapes, beware of the thorn. For sometimes the grape-cluster, springing from the root of the vine, finds its support in a common hedge; its branch, grows, becomes embedded among thorns, and the thorn bears other fruit than its own.
For the thorn has not been produced from the vine, but has become the resting-place of its runner. Make your inquiries only at the roots. Seek for the thorn-root, you will find it apart from the vine: seek the origin of the grape, and from the root of the vine it will be found to have sprung. And so, Moses' seat was the vine; the morals of the Pharisees were the thorns. Sound doctrine comes through the wicked, as the vine-branch in a hedge, a bunch of grapes among thorns. Gather carefully, so as in seeking the fruit not to tear your hand; and while you are to hear one speaking what is good, imitate him not when doing what is evil.
“What they tell you, do,”— gather the grapes; “but what they do, do not,”— beware of the thorns. Even through hirelings listen to the voice of the Shepherd, but be not hirelings yourselves, seeing you are members of the Shepherd. Yea, Paul himself, the holy apostle who said, “I have no one who has a brother's concern about you; for all seek their own, not the things which are Jesus Christ's,” draws a distinction in another place between hirelings and sons; and see what he says: “Some preach Christ even of envy and strife, and some also of good will: some of love, knowing that I am set for the defence of the gospel; but some also preach Christ of contention, not sincerely, supposing to add affliction to my bonds.”
These were hirelings who disliked the Apostle Paul. And why such dislike, but just because they were seeking after temporal things? But mark what he adds: “What then, notwithstanding, every way, whether in pretence or in truth, Christ is preached: and I therein do rejoice, yea, and will rejoice.” Christ is the truth: let the truth be preached in pretense by hirelings, let it be preached in truth by the children: the children are waiting patiently for the eternal inheritance of the Father, the hirelings are longing for, and in a hurry to get, the temporal pay of their employer.
For my part let me be shorn of the human glory, which I see such an object of envy to hirelings: and yet by the tongues both of hirelings and of children let the divine glory of Christ be published abroad, seeing that, “whether in pretense or in truth, Christ is preached.”
Source: Tractates on the Gospel of John (New Advent)