Charles Ellicott Commentary 1 Corinthians 2:9

"but as it is written, Things which eye saw not, and ear heard not, And [which] entered not into the heart of man, Whatsoever things God prepared for them that love him." — 1 Corinthians 2:9 (ASV)

As it is written. Where do the words that follow occur? They are not found as given here anywhere in the Old Testament. It has therefore been suggested (by Origen) that they are from some apocryphal book, or from a book that has been lost, as many are supposed to have been.

Chrysostom also suggests that it may be a reference not to a writing but to historical facts, as in Matthew 2:23. None of these explanations would justify using the phrase “it is written,” with which these words are introduced. In the apostolic writings, this phrase is confined to quotations from the Old Testament scriptures.

The phrase “it is written” is not used when words are taken from other sources (see, for example, Jude 1:9; Jude 1:14).

Although the words given here are not found in the same sequence in any Old Testament passage, there are still phrases scattered throughout the writings of Isaiah (Isaiah 65:17; see also Isaiah 62:15 in the LXX). These phrases could easily be joined together in memory and would resemble even verbally the passage as the Apostle wrote it here.

This is not the only instance where St. Paul seems to refer to the Old Testament scriptures in this way (see 1 Corinthians 1:19–20). He does so when he is not basing an argument on a particular sentence in the Scriptures but is merely making use of some thoughts or words from the Old Testament as an illustration of a truth he is emphasizing.