If mankind had seen Christ, if they had clearly identified with Him, the history of the world would be exceedingly different. They did not see because, as Paul writes here, they were not mature. Mature, in this context, means "converted." He contrasts those who are able to see and those who are not able to see. Those who are able to see are those who are spiritually mature.
Even though Christ quoted—and lived—the scriptures with which most of His audience were familiar, the people did not see God working through Him. So it has always been with God's servants. Christ was not the only one. Jesus Himself testifies that these people also "kill[ed] the prophets" (Matthew 23:34-37). It is unlikely that they would have killed the prophets if they clearly saw them as God's messengers. If they believed in God and were fearful of His authority and sovereignty over His creation, they would not have dared to do it! Nevertheless, it has always been this way: Some see and some do not see.
Paul says in I Corinthians 2:7 that God's ministers "speak the wisdom of God in a mystery." This mystery is not a puzzle that is difficult to solve but "a secret impossible to penetrate." As the apostle goes on to say in succeeding verses, the world is not "all there" upstairs because they do not have God's Spirit to help them penetrate the secret. Without this vital ingredient, it is no wonder that it accepts its own and rejects the truths of God.
Paul writes in verse 9, "But as it is written: 'Eye has not seen, nor ear heard, nor have entered into the heart of man the things which God has prepared for those who love Him.'" Many in the world believe that the things of God are "too great" for mere humans to comprehend. We really cannot "get it" or see it. Yet, the truth is so simple to those whose eyes are open that a child can understand. The carnal mind, however, is so blinded by traditions and habits of thinking that even Christians tend to reject the things of God—even though God has converted us.
The effect of this is something like the story about the three blind Indians who were led up to an elephant. Each man touched a different part of the great beast. One held the elephant's trunk, and when asked what it was, he said, "This is a snake." The second man, holding the elephant's tail, said, "This is a rope." The third man, feeling the elephant's leg, said, "This is a tree."
This is analogous to what happens in the world. The world can perceive bits and pieces of the truth, but they cannot put it all together and see the glory of God in its whole. They cannot see God as an intrinsic—absolutely necessary—part of a person's life. They cannot see how necessary the spiritual is!
If it is seen and if it is understood, then life begins to make sense. We begin to be able to see ourselves—a single, unique individual—as a part of the whole, the awesome plan and purpose that God is working out! Then, being able to see God gives direction to our life. So our eyes have seen and our ears have heard, and "the things which God has prepared for those who love Him" has entered into our hearts.