Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance, which is borne to man by the ministry of the angels.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic ministry sin
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“Reply Obj. 2: When man commits sin without being thereto instigated by the devil, he nevertheless becomes a child of the devil thereby, in so far as he imitates him who was the first to sin. Reply Obj. 3: Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance, which is borne to man by the ministry of the angels. For this reason the angels take part in all our good works: whereas all our sins are not due to the demons' instigation. Nevertheless there is no kind of sin which is not sometimes due to the demons' suggestion.” source