Considered only as regards the grade of nature, men can in no way be assumed into the angelic orders; for the natural distinction will always remain.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic distinction order
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

“On the contrary, The Lord says of the saints that, "they will be as the angels of God" (Matt. 22:30). I answer that, As above explained (AA. 4,7) , the orders of the angels are distinguished according to the conditions of nature and according to the gifts of grace. Considered only as regards the grade of nature, men can in no way be assumed into the angelic orders; for the natural distinction will always remain. In view of this distinction, some asserted that men can in no way be transferred to an equality with the angels; but this is erroneous, contradicting as it does the promise of Christ saying that the children of the resurrection will be equal to the angels in heaven (Luke 20:36).” source