Objection 1. It would seem that scandal is not a sin. For sins do not occur from necessity, since all sin is voluntary, as stated above (I-II, 74, A1,2). Now it is written (Matthew 18:7): "It must needs be that scandals come." Therefore scandal is not a sin.
Objection 2. Further, no sin arises from a sense of dutifulness, because "a good tree cannot bring forth evil fruit" (Matthew 7:18). But scandal may come from a sense of dutifulness, for Our Lord said to Peter (Matthew 16:23): "Thou art a scandal unto Me," in reference to which words Jerome says that "the Apostle's error was due to his sense of dutifulness, and such is never inspired by the devil." Therefore scandal is not always a sin.
Objection 3. Further, scandal denotes a stumbling. But he that stumbles does not always fall. Therefore scandal, which is a spiritual fall, can be without sin.
On the contrary, Scandal is "something less rightly said or done." Now anything that lacks rectitude is a sin. Therefore scandal is always with sin.
I answer that, As already said (1, ad 4), scandal is of two kinds, passive scandal in the person scandalized, and active scandal in the person who gives scandal, and so occasions a spiritual downfall. Accordingly passive scandal is always a sin in the person scandalized; for he is not scandalized except in so far as he succumbs to a spiritual downfall, and that is a sin.
Yet there can be passive scandal, without sin on the part of the person whose action has occasioned the scandal, as for instance, when a person is scandalized at another's good deed. On like manner active scandal is always a sin in the person who gives scandal, since either what he does is a sin, or if it only have the appearance of sin, it should always be left undone out of that love for our neighbor which binds each one to be solicitous for his neighbor's spiritual welfare; so that if he persist in doing it he acts against charity.
Yet there can be active scandal without sin on the part of the person scandalized, as stated above (1, ad 4).
Reply to Objection 1. These words, "It must needs be that scandals come," are to be understood to convey, not the absolute, but the conditional necessity of scandal; in which sense it is necessary that whatever God foresees or foretells must happen, provided it be taken conjointly with such foreknowledge, as explained in I, 14, 13, ad 3; I, 23, 6, ad 2.
Or we may say that the necessity of scandals occurring is a necessity of end, because they are useful in order that "they... who are reproved may be made manifest" (1 Corinthians 11:19).
Or scandals must needs occur, seeing the condition of man who fails to shield himself from sin. Thus a physician on seeing a man partaking of unsuitable food might say that such a man must needs injure his health, which is to be understood on the condition that he does not change his diet. On like manner it must needs be that scandals come, so long as men fail to change their evil mode of living.
Reply to Objection 2. In that passage scandal denotes any kind of hindrance: for Peter wished to hinder Our Lord's Passion out of a sense of dutifulness towards Christ.
Reply to Objection 3. No man stumbles spiritually, without being kept back somewhat from advancing in God's way, and that is at least a venial sin.
Source: Summa Theologica (New Advent)