I would say it might help to read it in context:
6 Howbeit we speak wisdom among them that are perfect: yet not the wisdom of this world, nor of the princes of this world, that come to nought:
7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
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.
10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
God's wisdom refered to in verse 7 is his plan for salvation. This was his own eternal secret that no one else could fathom, but in this new age of salvation he has graciously revealed it to us. Verses 9 & 10 (as Deut correctly pointed out) is taken from Isaiah.... and I believe that Paul uses this as an example of how the "new age" has changed things: God was "hidden", but is now "revealed" in Christ.