‘But as it is written, “Things which eye saw not, and ear heard not, and which did not enter into the heart of man, whatever things (those refer to) God prepared for those who love him”.'
He declares that Scripture reveals that what he has been describing is beyond human comprehension. It is describing what man could neither see, nor hear, nor know within. It therefore results in something that is naturally outside man's ability to understand. Yet it speaks of what God has prepared for those who love Him. And he goes on to say that it is revealed by God's own Spirit coming to man's spirit, if they receive Him, and making all supernaturally known.
‘As it is written.' Again Paul intends to reinforce his argument from the authoritative word of God.
The verse in mind is Isaiah 64:4 possibly amplified by Isaiah 65:16 c (LXX). Isaiah 64:4 reads in the Hebrew, ‘From of old men have not heard, nor perceived by the ear, neither has the eye seen, a God beside you Who works for those who wait for Him.' In LXX it reads, ‘From of old we have not heard, nor have our eyes seen a God beside you, and your works which you will perform for those who wait for mercy'. Isaiah 65:16 c LXX reads ‘neither shall they at all come into their mind' (Hebrew ‘nor come into mind').
As regularly (compare 1 Corinthians 1:19; 1 Corinthians 1:31) Paul may well be making a deliberate paraphrase in order to specifically apply the verse or verses (compare the same method in Mark 1:2-3) to the situation, for the point he is bringing out is that God has done a new thing for His own which is beyond anything man has known or seen, He is working for them in a new way, just as He promised in the days of Isaiah. The change from ‘wait for Him' to ‘love Him' is in part simply a change of emphasis, for those who wait for Him are those who love Him, and in part a declaration that there has been a moving forward. They no longer wait lovingly but love Him because He has acted, because of what He has done in the cross. Paul is concerned that there be a full response to the significance of the cross. To Paul Christians are those who supremely love God (Romans 8:28).
Origen suggested that this actual wording was as found in the Apocalypse of Elijah, but that is unknown to us and it may well be that that apocalyptic writing as known to Origen was quoting from Paul, just as Clement of Rome may have had Paul's quotation in mind when he writes ‘For [the Scripture] says, "Eye has not seen, nor ear heard, neither have entered into the heart of man, the things which He has prepared for them that wait for Him”.' Alternately some have suggested that they all obtained it from a jointly known source such as a Jewish/apocalyptic collection of verses not known to us. (Exact quotation was more difficult in those days due to shortage of manuscripts and the difficulty in consulting them, and anthologies would often be used, just as we use different versions).
But the significance of the words is the same. What God will do is beyond what man has ever known, for God will act on behalf of those who love Him, who trust Him, who wait for Him, in a way beyond telling.