If mankind had seen Christ, if they had clearly identified with Him, the history of the world would be exceedingly different. But they did not see because, as Paul writes here, they were not mature. Mature, in this context, means converted. It does not always mean exactly that, but in this context it does. There is a contrast between those who are able to see and those who are not able to see. And, those who are able to see are, in this context, those who are mature.
Even though Christ quoted - and lived - the scriptures that many in His audience were clearly familiar with, they did not see God working through Him. This is the way that it has always been with God's servants. Christ was not the only one. Christ Himself testified that those people killed the prophets. Now, I think that they would not have killed the prophets if they clearly saw them as being messengers from God. If they clearly believed in God and were fearful of His authority over His creation, His governance of it, they would not have dared to have done that! That is the way it has always been - there are those who see and there are those who do not see.
Paul testifies here that people are looking into a mystery. This mystery is not a puzzle that is difficult to solve, but a secret impossible to penetrate. So the world, as a result, is not all there upstairs. So it accepts its own and rejects the truths of God.