Objection 1. It would seem that before the public denunciation witnesses ought not to be brought forward. For secret sins ought not to be made known to others, because by so doing "a man would betray his brother's sins instead of correcting them," as Augustine says (De Verb. Dom. xvi, 7). Now by bringing forward witnesses one makes known a brother's sin to others. Therefore in the case of secret sins one ought not to bring witnesses forward before the public denunciation.
Objection 2. Further, man should love his neighbor as himself. Now no man brings in witnesses to prove his own secret sin. Neither therefore ought one to bring forward witnesses to prove the secret sin of our brother.
Objection 3. Further, witnesses are brought forward to prove something. But witnesses afford no proof in secret matters. Therefore it is useless to bring witnesses forward in such cases.
Objection 4. Further, Augustine says in his Rule that "before bringing it to the notice of witnesses... it should be put before the superior." Now to bring a matter before a superior or a prelate is to tell the Church. Therefore witnesses should not be brought forward before the public denunciation.
On the contrary, Our Lord said (Matthew 18:16): "Take with thee one or two more, that in the mouth of two," etc.
I answer that, The right way to go from one extreme to another is to pass through the middle space. Now Our Lord wished the beginning of fraternal correction to be hidden, when one brother corrects another between this one and himself alone, while He wished the end to be public, when such a one would be denounced to the Church. Consequently it is befitting that a citation of witnesses should be placed between the two extremes, so that at first the brother's sin be indicated to a few, who will be of use without being a hindrance, and thus his sin be amended without dishonoring him before the public.
Reply to Objection 1. Some have understood the order of fraternal correction to demand that we should first of all rebuke our brother secretly, and that if he listens, it is well; but if he listen not, and his sin be altogether hidden, they say that we should go no further in the matter, whereas if it has already begun to reach the ears of several by various signs, we ought to prosecute the matter, according to Our Lord's command.
But this is contrary to what Augustine says in his Rule that "we are bound to reveal" a brother's sin, if it "will cause a worse corruption in the heart." Wherefore we must say otherwise that when the secret admonition has been given once or several times, as long as there is probable hope of his amendment, we must continue to admonish him in private, but as soon as we are able to judge with any probability that the secret admonition is of no avail, we must take further steps, however secret the sin may be, and call witnesses, unless perhaps it were thought probable that this would not conduce to our brother's amendment, and that he would become worse: because on that account one ought to abstain altogether from correcting him, as stated above (Article 6).
Reply to Objection 2. A man needs no witnesses that he may amend his own sin: yet they may be necessary that we may amend a brother's sin. Hence the comparison fails.
Reply to Objection 3. There may be three reasons for citing witnesses. First, to show that the deed in question is a sin, as Jerome says: secondly, to prove that the deed was done, if repeated, as Augustine says (in his Rule): thirdly, "to prove that the man who rebuked his brother, has done what he could," as Chrysostom says (Hom. in Matth. lx).
Reply to Objection 4. Augustine means that the matter ought to be made known to the prelate before it is stated to the witnesses, in so far as the prelate is a private individual who is able to be of more use than others, but not that it is to be told him as to the Church, i.e. as holding the position of judge.
Source: Summa Theologica (New Advent)