These are the Summa Theologiae passages I referenced in Part 2 of my article: Is it Virtuous to Criticize the Pope? (For passages referenced in Part 1, click here.)
I’ve provided links to New Advent in case you want to see them in context. For your ease of reading, I’ve done the following:
underlined the two terms when there's a distinction
bold-faced the topic word of the passage
added some formatting and numbering.
Scandal, and Piety
from Secunda Secundae (II-II)
Scandal
Question 43
Article 1
Reply to Objection 4. Another's words or deed may be the cause of another's sin in two ways, directly and accidentally. Directly, when a man either intends, by his evil word or deed, to lead another man into sin, or, if he does not so intend, when his deed is of such a nature as to lead another into sin: for instance, when a man publicly commits a sin or does something that has an appearance of sin. On this case he that does such an act does, properly speaking, afford an occasion of another's spiritual downfall, wherefore his act is called "active scandal."
One man's word or deed is the accidental cause of another's sin, when he neither intends to lead him into sin, nor does what is of a nature to lead him into sin, and yet this other one, through being ill-disposed, is led into sin, for instance, into envy of another's good, and then he who does this righteous act, does not, so far as he is concerned, afford an occasion of the other's downfall, but it is this other one who takes the occasion according to Romans 7:8: "Sin taking occasion by the commandment wrought in me all manner of concupiscence." Wherefore this is "passive," without "active scandal," since he that acts rightly does not, for his own part, afford the occasion of the other's downfall.
…scandal is a special kind of sin, because thereby a man intends a special harm to his neighbor, and it is directly opposed to fraternal correction, whereby a man intends the removal of a special kind of harm.
Reply to Objection 2. In the opinion of Augustine (Ep. xxviii, xl, lxxxii) and of Paul also, Peter sinned and was to be blamed, in withdrawing from the gentiles in order to avoid the scandal of the Jews, because he did this somewhat imprudently, so that the gentiles who had been converted to the faith were scandalized. Nevertheless Peter's action was not so grave a sin as to give others sufficient ground for scandal. Hence they were guilty of passive scandal, while there was no active scandal in Peter.
Reply to Objection 3. The venial sins of the perfect consist chiefly in sudden movements, which being hidden cannot give scandal. If, however, they commit any venial sins even in their external words or deeds, these are so slight as to be insufficient in themselves to give scandal.
I answer that, … Now a distinction must be made in spiritual goods. For some of them are necessary for salvation, and cannot be foregone without mortal sin: and it is evident that no man ought to commit a mortal sin, in order to prevent another from sinning, because according to the order of charity, a man ought to love his own spiritual welfare more than another's. Therefore one ought not to forego that which is necessary for salvation, in order to avoid giving scandal.
Again a distinction seems necessary among spiritual things which are not necessary for salvation:
because the scandal which arises from such things sometimes proceeds from malice, for instance when a man wishes to hinder those spiritual goods by stirring up scandal. This is the "scandal of the Pharisees," who were scandalized at Our Lord's teaching: and Our Lord teaches (Mt. 15:14) that we ought to treat such like scandal with contempt.
Sometimes scandal proceeds from weakness or ignorance, and such is the "scandal of little ones." In order to avoid this kind of scandal, spiritual goods ought to be either concealed, or sometimes even deferred (if this can be done without incurring immediate danger), until the matter being explained the scandal cease. If, however, the scandal continue after the matter has been explained, it would seem to be due to malice, and then it would no longer be right to forego that spiritual good in order to avoid such like scandal.
Reply to Objection 1: …But if it is evident that the infliction of punishment will result in more numerous and more grievous sins being committed, the infliction of punishment will no longer be a part of justice. It is in this sense that Augustine is speaking, when, to wit, the excommunication of a few threatens to bring about the danger of a schism, for in that case it would be contrary to the truth of justice to pronounce excommunication.
Reply to Objection 2. With regard to a man's doctrine two points must be considered, namely, the truth which is taught, and the act of teaching.
The first of these is necessary for salvation, to wit, that he whose duty it is to teach should not teach what is contrary to the truth, and that he should teach the truth according to the requirements of times and persons: wherefore on no account ought he to suppress the truth and teach error in order to avoid any scandal that might ensue.
But the act itself of teaching is one of the spiritual almsdeeds, as stated above (II-II:32:2), and so the same is to be said of it as of the other works of mercy, of which we shall speak further on (Reply to Objection 4).
Reply to Objection 3. As stated above (II-II:33:1), fraternal correction aims at the correction of a brother, wherefore it is to be reckoned among spiritual goods in so far as this end can be obtained, which is not the case if the brother be scandalized through being corrected. And so, if the correction be omitted in order to avoid scandal, no spiritual good is foregone.
Reply to Objection 4. The truth of life, of doctrine, and of justice comprises not only whatever is necessary for salvation, but also whatever is a means of obtaining salvation more perfectly, according to 1 Corinthians 12:31: "Be zealous for the better gifts." Wherefore neither the counsels nor even the works of mercy are to be altogether omitted in order to avoid scandal; but sometimes they should be concealed or deferred, on account of the scandal of the little ones, as stated above.
Piety
Question 81
Reply to Objection 2. By the one same act man both serves and worships God, for worship regards the excellence of God, to Whom reverence is due: while service regards the subjection of man who, by his condition, is under an obligation of showing reverence to God.
I answer that, We pay God honor and reverence, not for His sake (because He is of Himself full of glory to which no creature can add anything), but for our own sake, because by the very fact that we revere and honor God, our mind is subjected to Him; wherein its perfection consists, since a thing is perfected by being subjected to its superior.
Question 144
I answer that, Since reproach is opposed to honor, just as honor denotes attestation to someone's excellence, especially the excellence which is according to virtue, so too reproach, the fear of which is shamefacedness, denotes attestation to a person's defect, especially that which results from sin.
Question 5
…the species of every habit depends on the formal aspect of the object, without which the species of the habit cannot remain. Now the formal object of faith is the First Truth, as manifested in Holy Writ and the teaching of the Church, which proceeds from the First Truth. Consequently whoever does not adhere, as to an infallible and Divine rule, to the teaching of the Church, which proceeds from the First Truth manifested in Holy Writ, has not the habit of faith, but holds that which is of faith otherwise than by faith. Even so, it is evident that a man whose mind holds a conclusion without knowing how it is proved, has not scientific knowledge, but merely an opinion about it.
Question 99
I answer that, Wherever we find a special aspect of deformity, there must needs be a special sin; because the species of a thing is derived chiefly from its formal aspect, and not from its matter or subject. Now in sacrilege we find a special aspect of deformity, namely, the violation of a sacred thing by treating it irreverently. Hence it is a special sin.