When man is in a state of grace, he can avoid all mortal sins, and each single one
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic sin state
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/19950/pg19950-images.html

Context

“Consequently it is necessary to have a certain virtual displeasure, so that, for instance, a man's affections so tend to God and Divine things, that whatever might happen to him to hamper that tendency would be displeasing to him, and would grieve him, were he to commit it, even though he were not to think of it actually: and this is not sufficient for the remission of mortal sin, except as regards those sins which he fails to remember after a careful examination. Reply Obj. 1: When man is in a state of grace, he can avoid all mortal sins, and each single one; and he can avoid each single venial sin, but not all, as was explained in the Second Part (I-II, Q. 74, A. 8, ad 2; Q. 109, A. 8) . Consequently penance for mortal sins requires man to purpose abstaining from mortal sins, all and each;” source