Since therefore the created light of glory received into any created intellect cannot be infinite, it is clearly impossible for any created intellect to know God in an infinite degree. Hence it is impossible that it should comprehend God.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic glory God
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

“But God, whose being is infinite, as was shown above (Q. 7) , is infinitely knowable. Now no created intellect can know God infinitely. For the created intellect knows the Divine essence more or less perfectly in proportion as it receives a greater or lesser light of glory. Since therefore the created light of glory received into any created intellect cannot be infinite, it is clearly impossible for any created intellect to know God in an infinite degree. Hence it is impossible that it should comprehend God. Reply Obj. 1: "Comprehension" is twofold: in one sense it is taken strictly and properly, according as something is included in the one comprehending; and thus in no way is God comprehended either by intellect, or in any other way; forasmuch as He is infinite and cannot be included in any finite being; so that no finite being can contain Him infinitely, in the degree of His own infinity.” source