1 And when I came to you, brothers, I did not come with excellence of speech or of wisdom, declaring to you the testimony of God. 2 For I did not determine to know anything among you, except Jesus Christ and him crucified. 3 And I was with you in weakness, in fear, and in much trembling. 4 And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power, 5 So that your faith should not stand in the wisdom of men, but in the power of God.
6 However, we do speak wisdom among those who are mature, but not the wisdom of this world, nor of the princes of this world, which are coming to nothing. 7 But we speak the wisdom of God in a mystery, even the hidden wisdom which God ordained before the ages to our glory, 8 Which none of the princes of this age knew. For had they known it, they would not have crucified the Lord of glory.
9 But as it is written, "'Eye has not seen, nor ear heard,' neither have entered into the heart of man, 'the things which God has prepared for those who love him.'"
10 But God has revealed them to us by his Spirit, for the Spirit searches all things, even the deep things of God. 11 For what man knows the things of a man except the spirit of man which is in him? Even so, the things of God no man knows except the Spirit of God. 12 Now we have not received the spirit of the world, but the Spirit which is from God so that we may know the things that are freely given to us by God, 13 Which things we also speak, not in the words which man's wisdom teaches, but which the Holy Spirit teaches, comparing spiritual things with spiritual.
14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him, neither can he know them because they are spiritually discerned. 15 But he who is spiritual judges all things, yet he himself is judged by no man. 16 "For who has known the mind of the Lord so that he may instruct him?" But we have the mind of Christ.
[v.9] - Quoting Isaiah 64:4 (paraphrased). From John Calvin's Commentary: "As to what follows respecting the entering of these things into the heart of man, though the expression is not made use of by the Prophet, it does not differ materially from the clause 'besides thee.' For in ascribing this knowledge to God alone, he excludes from it not merely the bodily senses of men, but also the entire faculty of the understanding. While, therefore, the Prophet makes mention only of sight and hearing, he includes at the same time by implication all the faculties of the soul. And without doubt these are the two instruments by which we attain the knowledge of those things that find their way into the understanding. In using the expression 'them that love him,' he has followed the Greek interpreters, who have translated it in this way from having been misled by the resemblance between one letter and another; but as that did not affect the point in hand, he did not choose to depart from the common reading, as we frequently have occasion to observe how closely he follows the received version. Though the words, therefore, are not the same, there is no real difference of meaning." (Quotes added.)
[v.16] - Quoting Isaiah 40:13 (LXX).