1 Corinthians 2:9

But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him (KJV)

Then: The Corinthians probably understood this to mean that no one yet knows nor can even dream of the things God has in store for those who love him. They also knew the original references which Paul was using to talk to them. For readers, the imagery is striking: The things that no eye has ever seen, no ear has ever heard appeal to imaginations. God's world and work far surpasses human ability to see, hear, and imagine. The thoughts that the Lord has more to teach them would have encouraged the Corinthians.

Now: Grammar for the verse confuses the commentators because there is no main verb (Thiselton, p. 248-250). That difficulty compounds in trying to locate the exact source of Paul's quote - Maybe Isaiah 64:4 and maybe 65:16. Baker (p. 45) states that the Gospel of Thomas has Isaiah 64:4 reading with words closer to that of Paul's than those in the septuagint. The message still inspires readers today to know that the Lord is great and we can rejoice in being able to depend upon his greatness.

Extra source - The First Epistle to the Corinthians: a commentary on the Greek text by Anthony C. Thiselton (Eerdman, 2000) online at http://books.google.com/books?id=aNkcqC9bdAMC&dq=Corinthians+and+Thiselton&printsec=frontcover&source=bl&ots=eoboP0fSHQ&sig=DLgK1fuY0qIDxrDL1oog1wwHZ9w&hl=en&ei=MfofS9XKLJGHlAfY_8j7Cw&sa=X&oi=book_result&ct=result&resnu
Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.