1 Corinthians 1:18 (ESV) … “For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God.”
Paul began by explaining his straightforward manner of preaching in terms of the ways believers and unbelievers view the gospel. On the one hand, those who are perishing (unbelievers) live according to the standards of sinful human wisdom, and therefore wrongly conclude that the message of the cross is foolishness.
On the other hand, the Holy Spirit changes the perspectives of those who are being saved. Those who follow Christ rightly perceive that the Cross is not foolishness, but the power of God bringing salvation from sin and death.
Paul pressed this radical antithesis between the outlook of believers and unbelievers in order to remind the Corinthians that the way of Christ does not rely on sinful human wisdom. By this means, he pointed out that they thought and acted like unbelievers when they were arrogantly and pridefully divided. 
When those who are perishing permit the gospel of the cross to enlighten them, they no longer count this gospel to be foolishness but the power of God and when they embrace it, they leave the ranks of those who are perishing and enter the ranks of those who are being saved.