Objection 1. It would seem that a sinner ought to reprove a wrongdoer. For no man is excused from obeying a precept by having committed a sin. But fraternal correction is a matter of precept, as stated above (Article 2). Therefore it seems that a man ought not to forbear from such like correction for the reason that he has committed a sin.
Objection 2. Further, spiritual almsdeeds are of more account than corporal almsdeeds. Now one who is in sin ought not to abstain from administering corporal alms. Much less therefore ought he, on account of a previous sin, to refrain from correcting wrongdoers.
Objection 3. Further, it is written (1 John 1:8): "If we say that we have no sin, we deceive ourselves." Therefore if, on account of a sin, a man is hindered from reproving his brother, there will be none to reprove the wrongdoer. But the latter proposition is unreasonable: therefore the former is also.
On the contrary, Isidore says (De Summo Bono iii, 32): "He that is subject to vice should not correct the vices of others." Again it is written (Romans 2:1): "Wherein thou judgest another, thou condemnest thyself. For thou dost the same things which thou judgest."
I answer that, As stated above (3, ad 2), to correct a wrongdoer belongs to a man, in so far as his reason is gifted with right judgment. Now sin, as stated above (I-II, 85, 1,2), does not destroy the good of nature so as to deprive the sinner's reason of all right judgment, and in this respect he may be competent to find fault with others for committing sin. Nevertheless a previous sin proves somewhat of a hindrance to this correction, for three reasons. First because this previous sin renders a man unworthy to rebuke another; and especially is he unworthy to correct another for a lesser sin, if he himself has committed a greater. Hence Jerome says on the words, "Why seest thou the mote?" etc. (Matthew 7:3): "He is speaking of those who, while they are themselves guilty of mortal sin, have no patience with the lesser sins of their brethren."
Secondly, such like correction becomes unseemly, on account of the scandal which ensues therefrom, if the corrector's sin be well known, because it would seem that he corrects, not out of charity, but more for the sake of ostentation. Hence the words of Matthew 7:4, "How sayest thou to thy brother?" etc. are expounded by Chrysostom [Hom. xvii in the Opus Imperfectum falsely ascribed to St. John Chrysostom] thus: "That is--'With what object?' Out of charity, think you, that you may save your neighbor?" No, "because you would look after your own salvation first. What you want is, not to save others, but to hide your evil deeds with good teaching, and to seek to be praised by men for your knowledge."
Thirdly, on account of the rebuker's pride; when, for instance, a man thinks lightly of his own sins, and, in his own heart, sets himself above his neighbor, judging the latter's sins with harsh severity, as though he himself were just man. Hence Augustine says (De Serm. Dom. in Monte ii, 19): "To reprove the faults of others is the duty of good and kindly men: when a wicked man rebukes anyone, his rebuke is the latter's acquittal." And so, as Augustine says (De Serm. Dom. in Monte ii, 19): "When we have to find fault with anyone, we should think whether we were never guilty of his sin; and then we must remember that we are men, and might have been guilty of it; or that we once had it on our conscience, but have it no longer: and then we should bethink ourselves that we are all weak, in order that our reproof may be the outcome, not of hatred, but of pity.
But if we find that we are guilty of the same sin, we must not rebuke him, but groan with him, and invite him to repent with us." It follows from this that, if a sinner reprove a wrongdoer with humility, he does not sin, nor does he bring a further condemnation on himself, although thereby he proves himself deserving of condemnation, either in his brother's or in his own conscience, on account of his previous sin.
Hence the Replies to the Objections are clear.
Source: Summa Theologica (New Advent)