God's way of thinking is not like the world's way (normal human wisdom), but God offers eternal life while the world cannot. We can spend a lifetime accumulating human wisdom & never learn how to have a personal relationship with God. We must come to Christ to learn this important truth.
Many Jews thought the Good News of Jesus Christ was foolish because they thought the Messiah would be a conquering King accompanied by signs & miracles. Jesus had not restored David's throne as they expected. Greeks, too, considered the Gospel foolish; they did not believe in a bodily resurrection, they did not see in Jesus the powerful characteristics of their mythological gods; & they thought no reputable person would be crucified.
The Good News of Jesus Christ still sounds foolish to many. Our society worships power, influence, & wealth. Jesus came as a humble poor servant, & he offers his Kingdom to those with faith, not works. This looks foolish to the world.
The message of Christ's death for sins sounds foolish to those who don't believe. Death seems to be the end of the road, the ultimate weakness. But Jesus did not stay dead. His resurrection shows his power even over death, & he will save us from eternal death, & give us everlasting life if we trust him as Saviour & Lord. This sounds so simple that many people won't accept it. They try other ways to obtain eternal life (being good, being wise, etc.), but their attempts will not work. The "foolish" people who simply accept Christ's offer are actually the wisest of all, because they alone will live eternally with God.
Reference summary used from the Life Application Bible KJV, Tyndale House Publishers Inc., Wheaton, Illinois