For every sin that is contrary to charity is a mortal sin, as stated above (Q. 24, A. 12; Q. 35, A. 3). But scandal is contrary to charity, as stated above (AA. 2, 3). Therefore scandal is a mortal sin.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic sin charity
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“3: Scandal does not derive the species of a special sin from the circumstance in question, but from the intention of the end, as stated above. _______________________
FOURTH ARTICLE [II-II, Q. 43, Art. 4]
Whether Scandal Is a Mortal Sin?
Objection 1: It would seem that scandal is a mortal sin. For every sin that is contrary to charity is a mortal sin, as stated above (Q. 24, A. 12; Q. 35, A. 3). But scandal is contrary to charity, as stated above (AA. 2, 3). Therefore scandal is a mortal sin. Obj. 2: Further, no sin, save mortal sin, deserves the punishment of eternal damnation. But scandal deserves the punishment of eternal damnation, according to Matt. 18:6: "He that shall scandalize one of these little ones, that believe in Me, it were better for him that a mill-stone should be hanged about his neck, and that he should be drowned in the depth of the sea."” source