Objection 1. It would seem that active scandal can be found in the perfect. For passion is the effect of action. Now some are scandalized passively by the words or deeds of the perfect, according to Matthew 15:12: "Dost thou know that the Pharisees, when they heard this word, were scandalized?" Therefore active scandal can be found in the perfect.
Objection 2. Further, Peter, after receiving the Holy Ghost, was in the state of the perfect. Yet afterwards he scandalized the gentiles: for it is written (Galatians 2:14): "When I saw that they walked not uprightly unto the truth of the Gospel, I said to Cephas," i.e. Peter, "before them all: If thou being a Jew, livest after the manner of the gentiles, and not as the Jews do, how dost thou compel the gentiles to live as do the Jews?" Therefore active scandal can be in the perfect.
Objection 3. Further, active scandal is sometimes a venial sin. But venial sins may be in perfect men. Therefore active scandal may be in perfect men.
On the contrary, Active scandal is more opposed to perfection, than passive scandal. But passive scandal cannot be in the perfect. Much less, therefore, can active scandal be in them.
I answer that, Active scandal, properly so called, occurs when a man says or does a thing which in itself is of a nature to occasion another's spiritual downfall, and that is only when what he says or does is inordinate. Now it belongs to the perfect to direct all their actions according to the rule of reason, as stated in 1 Corinthians 14:40: "Let all things be done decently and according to order"; and they are careful to do this in those matters chiefly wherein not only would they do wrong, but would also be to others an occasion of wrongdoing. And if indeed they fail in this moderation in such words or deeds as come to the knowledge of others, this has its origin in human weakness wherein they fall short of perfection. Yet they do not fall short so far as to stray far from the order of reason, but only a little and in some slight matter: and this is not so grave that anyone can reasonably take therefrom an occasion for committing sin.
Reply to Objection 1. Passive scandal is always due to some active scandal; yet this active scandal is not always in another, but in the very person who is scandalized, because, to wit, he scandalizes himself.
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.
Source: Summa Theologica (New Advent)