Objection 1. It would seem that passive scandal may happen even to the perfect. For Christ was supremely perfect: and yet He said to Peter (Matthew 16:23): "Thou art a scandal to Me." Much more therefore can other perfect men suffer scandal.
Objection 2. Further, scandal denotes an obstacle which is put in a person's spiritual way. Now even perfect men can be hindered in their progress along the spiritual way, according to 1 Thessalonians 2:18: "We would have come to you, I Paul indeed, once and again; but Satan hath hindered us." Therefore even perfect men can suffer scandal.
Objection 3. Further, even perfect men are liable to venial sins, according to 1 John 1:8: "If we say that we have no sin, we deceive ourselves." Now passive scandal is not always a mortal sin, but is sometimes venial, as stated above (Article 4). Therefore passive scandal may be found in perfect men.
On the contrary, Jerome, in commenting on Matthew 18:6, "He that shall scandalize one of these little ones," says: "Observe that it is the little one that is scandalized, for the elders do not take scandal."
I answer that, Passive scandal implies that the mind of the person who takes scandal is unsettled in its adherence to good. Now no man can be unsettled, who adheres firmly to something immovable. The elders, i.e. the perfect, adhere to God alone, Whose goodness is unchangeable, for though they adhere to their superiors, they do so only in so far as these adhere to Christ, according to 1 Corinthians 4:16: "Be ye followers of me, as I also am of Christ." Wherefore, however much others may appear to them to conduct themselves ill in word or deed, they themselves do not stray from their righteousness, according to Psalm 124:1: "They that trust in the Lord shall be as Mount Sion: he shall not be moved for ever that dwelleth in Jerusalem." Therefore scandal is not found in those who adhere to God perfectly by love, according to Psalm 118:165: "Much peace have they that love Thy law, and to them there is no stumbling-block [scandalum]."
Reply to Objection 1. As stated above (2, ad 2), in this passage, scandal is used in a broad sense, to denote any kind of hindrance. Hence Our Lord said to Peter: "Thou art a scandal to Me," because he was endeavoring to weaken Our Lord's purpose of undergoing His Passion.
Reply to Objection 2. Perfect men may be hindered in the performance of external actions. But they are not hindered by the words or deeds of others, from tending to God in the internal acts of the will, according to Romans 8:38-39: "Neither death, nor life... shall be able to separate us from the love of God."
Reply to Objection 3. Perfect men sometimes fall into venial sins through the weakness of the flesh; but they are not scandalized (taking scandal in its true sense), by the words or deeds of others, although there can be an approach to scandal in them, according to Psalm 72:2: "My feet were almost moved."
Source: Summa Theologica (New Advent)