What does Paul mean when he says 'as it is written'?
When Paul says 'as it is written,' he is pointing back to an Old Testament reference. The phrase introduces a quotation from earlier Scripture. Paul uses it to connect his teaching to what was already written, as seen in his quotation of verse 9 about what God has prepared for those who love Him.
Les explains that when Paul says 'as it is written,' he is taking us back to an Old Testament reference. In this case, Paul quotes the truth that eye has not seen, nor ear heard, neither has it entered into the heart of man the things which God has prepared for them that love Him. Les notes that this verse gives us a whole train load if we think on it, because we cannot begin to comprehend what God has already prepared for those who love Him. He encourages believers to get excited about their eternity, since it is already there, not something God will do someday. Les also contrasts this with present-day Christendom, where there is much talk about heaven but little about the lake of fire and eternal doom.
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.