2 Corinthians 4:17-18
“For the light affliction,” he says, “which is for the moment, works more and more exceedingly an eternal weight of glory; while we look not at the things which are seen, but at the things which are not seen.”
Having closed the question by a reference to hope, (and, as he said in his Epistle to the Romans, “We are saved by hope, but hope that is seen is not hope;” establishing the same point here also,) he sets side by side the things present with the things to come, the momentary with the eternal, the light with the weighty, the affliction with the glory. And neither is he content with this, but he adds another expression, doubling it and saying, “more and more exceedingly.” Next he also shows the mode how so great afflictions are light. How then light? “While we look not at the things that are seen, but at the things that are not seen.” So will both this present be light and that future great, if we withdraw ourselves from the things that are seen. “For the things that are seen are temporal.” Therefore the afflictions are so too. “But the things that are not seen are eternal.” Therefore the crowns are so also. And he said not the afflictions are so, but “the things that are seen;” all of them, whether punishment or rest, so that we should be neither puffed up by the one nor overborne by the other. And therefore when speaking of the things to come, he said not the kingdom is eternal; but, “the things which are not seen are eternal,” whether they be a kingdom, or again punishment; so as both to alarm by the one and to encourage by the other.
Source: Homilies on Second Corinthians (New Advent)