Man can avoid each but not every act of sin, except by grace, as stated above. Nevertheless, since it is by his own shortcoming that he does not prepare himself to have grace, the fact that he cannot avoid sin without grace does not excuse him from sin.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic sin excuse
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“But because a man cannot always have this premeditation, it cannot help occurring that he acts in accordance with his will turned aside from God, unless, by grace, he is quickly brought back to the due order. Reply Obj. 1: Man can avoid each but not every act of sin, except by grace, as stated above. Nevertheless, since it is by his own shortcoming that he does not prepare himself to have grace, the fact that he cannot avoid sin without grace does not excuse him from sin. Reply Obj. 2: Correction is useful "in order that out of the sorrow of correction may spring the wish to be regenerate; if indeed he who is corrected is a son of promise, in such sort that whilst the noise of correction is outwardly resounding and punishing, God by hidden inspirations is inwardly causing to will,"” source