I answer that, It is impossible for an effect to be stronger than its cause. Now every hatred arises from some love as its cause, as above stated (A. 2). Therefore it is impossible for hatred to be stronger than love absolutely.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love state
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

“On the contrary, Good is stronger than evil; because "evil does nothing except in virtue of good," as Dionysius says (Div. Nom. iv) . But hatred and love differ according to the difference of good and evil. Therefore love is stronger than hatred.
I answer that, It is impossible for an effect to be stronger than its cause. Now every hatred arises from some love as its cause, as above stated (A. 2). Therefore it is impossible for hatred to be stronger than love absolutely. But furthermore, love must needs be, absolutely speaking, stronger than hatred. Because a thing is moved to the end more strongly than to the means. Now turning away from evil is directed as a means to the gaining of good.” source