for God cannot require from man more than man can do, and since He is more inclined to be merciful than to punish, as He lays the act of sin to man's charge, so He ought to credit him with the contrary act.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic sin God
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

“He could also have restored it without assuming flesh. Obj. 2: Further, for the restoration of human nature, which had fallen through sin, nothing more is required than that man should satisfy for sin. Now man can satisfy, as it would seem, for sin; for God cannot require from man more than man can do, and since He is more inclined to be merciful than to punish, as He lays the act of sin to man's charge, so He ought to credit him with the contrary act. Therefore it was not necessary for the restoration of human nature that the Word of God should become incarnate. Obj. 3: Further, to revere God pertains especially to man's salvation; hence it is written” source