Διατριβη is from τριβω, "I rub." Those conducting a conversation rub against one another. But rubbing from a healthy person strengthens health, or at least does not disorder it. Rubbing from a sick person can communicate disease and infect. So it is here. Rub against a teacher of other doctrine in conversation, and you may be infected by his false teaching and fall ill with evil belief.
According to Saint Chrysostom, the words "evil discussions" have this meaning in the Apostle: "Just as sheep infected with mange communicate their disease to healthy sheep when they come into contact with them, so also do these evil men."
In Blessed Theodoret we read: "Those who have departed from the truth, the teachers of other doctrine, and who follow their own thoughts, attempt to teach what is improper. From this evil suspicions are born, and from them comes a certain destructive disease that infects neighbors. This is what the words 'evil discussions' signify. For as sick sheep grazing together with healthy ones communicate disease to them, so these men fill their neighbors with infection."
Oecumenius states directly: "Evil discussions are the transmission of the disease of evil dogmas, a metaphor taken from scabby sheep." Theophylact explains him: "As scabby sheep, rubbing among the healthy, infect them also with their disease, so teachers of other doctrine, rubbing through their conversations, παραδιατριβομενοι, with others, corrupt their minds and hearts with their evil belief."
Such fruit comes from them because they themselves are corrupt in mind. Such are all who do not approach the sound words of the Lord, unspoiled by perverse interpretation, and the doctrine piously held and preserved by the Church. Their mind, having accepted false principles and propositions, or dogmas, has become corrupt and has corrupted them.
They themselves are at fault. Why did they indulge their mind in taking up what was beyond its strength? They should have bowed it under the yoke of faith and made it obediently receive what was revealed and handed down, study it, and assimilate it. Instead, they allowed it to invent and compose the faith itself.
It became confused. But, having taken its own invention for unquestionable truth, it became corrupt itself and corrupted them. They became men corrupt in mind, infecting others with their own corruption. What else is to be expected from them? "A good man out of the good treasure brings forth good things, and an evil man out of the evil treasure brings forth evil things" (Mt 12:35).
They are corrupt in mind because they are deprived of the truth. This is the cause of the corruption of the mind. As the holding of truth is the health of the mind, so deprivation of truth is its sickness and corruption. Similarly, the keeping and preserving of the commandments is the health of the will, while turning aside from them is its sickness and corruption. Sympathy for the divine and heavenly, delight in it, and sweetness in it are the health of the heart. Deprivation of such sympathy, delight, and sweetness is its sickness and corruption.
Here the same thing happens as, for example, with berries. Leave them as they are, and they will soon spoil and rot. But fill them with sugar by boiling them in its solution, and they acquire power to resist corruption. So the mind, left as it is, soon spoils. Fill it with truth, and truth will communicate to it strength and power to resist every corruption.
Deprivation of truth produces two evils in the mind: either the darkness of ignorance or corruption by evil belief. Both are bad, but the latter is worse. Darkness, the deprivation of light, causes torpor and even death. Nothing living can live without light, material things without material light and spiritual things without spiritual light.
For the mind, truth is light. Without it, the mind grows numb or is numbed. This is a great evil. Yet for one who is numbed there may be hope that he will recover, come to himself, and live. But one corrupted by evil belief offers no such hope. He is inwardly and essentially damaged. Therefore he can be restored to the health proper to him by nature only by the power of God, who created natures. That is, he can be restored only by being remade through a special grace of God.
"Supposing that piety is gain." "Gain" is πορισμον. Πορος means having in abundance all the necessities of life. Πορισμος means the acquisition of these necessities and the means to that acquisition. By "piety" faith in general is understood. He says that they consider faith a means of acquisition and turn it into such a means.
This is the height of mental and moral corruption. Here there is no longer faith, and still less piety, or a heartfelt disposition in the spirit of faith. Meanwhile all their bustle concerns faith, and it is intense bustle, so that this matter does not pass without disputes.
Is this not where the disputes come from? As merchants dispute during a sale out of a desire to increase profit, so these teachers of other doctrine start disputes everywhere. Otherwise they will not be able to get anything and will have to go hungry.
"These disputants," writes Blessed Theophylact, "draw disciples to themselves, make use of them, and strive more and more in disputes so as to attract them more and more to themselves." This is, of course, so that they may acquire more from them.
"Withdraw from such." "He did not say, 'Engage them and wrestle,' but, 'Withdraw,' that is, after the first and second admonition (cf. Tit 3:10). For when and by what can those who fight for profit and money be persuaded?" (Blessed Theophylact).
"You will not persuade such men except by giving to them. Yet even by this you will not satisfy their thirst for gain. 'The eye of the covetous man,' says Scripture, 'is not satisfied with his portion' (cf. Sir 14:9). Therefore one must turn away from them as incorrigible" (Saint Chrysostom).