God's mercy is more powerful than man's, in that it moves man's will to repent, which man's mercy cannot do.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic mercy 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

“On the other hand, in the case of an adult, in whom there are actual sins, which consist in an actual disorder of the will, there is no remission of sins, even in Baptism, without an actual change of the will, which is the effect of Penance. Reply Obj. 2: This argument takes Penance as a sacrament. Reply Obj. 3: God's mercy is more powerful than man's, in that it moves man's will to repent, which man's mercy cannot do. _______________________ THIRD ARTICLE [III, Q. 86, Art. 3] Whether by Penance One Sin Can Be Pardoned Without Another? Objection 1: It would seem that by Penance one sin can be pardoned without another.” source