1 Corinthians 1:23 NKJV - "But we preach Christ crucified, to the Jews a stumbling block and to the Greeks foolishness."
To start with Paul is not saying that when he preaches he only concentrates on the crucifixion of Christ. No! He is simply reinforcing what he has been saying since verse 18 that the "preaching of the cross is foolishness" to so many. He focused on the "message of the cross" as the focal point of salvation. Sure, he knows Christ rose from the dead and emphasizes that in many ways throughout his letters (epistles). But Paul wants to make clear that God doesn't follow the so-called wisdom of mankind which would suggest that victory was far better than death. Most of the world would have said then and still say now, "let Him come down from the cross and we will believe!" To the world that would have been a far greater "WOW Factor" for people to get excited about.
Paul is saying in this verse that the crucifixion of Christ makes a Jew wonder how could it possibly be true that their Messiah would die, and that, die a terrible death like crucifixion! "It just shouldn't be!" is their reaction to this teaching. They were looking for a Messiah to set everything straight and make the Jewish nation the greatest on earth. Of course, we know that their minds were thinking of their political oppression far more than their sinful oppression.
To the Gentile, Paul says they just think it's utter nonsense to think that a man dying on a wooden cross 2,000 years ago should have any bearing on people's lives today. But that did not change Paul's preaching! He knew that the Messiah had to die a cruel death to save the Jew, and that Christ's death on the cross was the "wisest thing" God could do for all mankind! Remember, Christ's death guarantees eternal life for those who believe on Him! "He died that man might live!"
Between 1 Corinthians 1:18 and 23 we see that our preaching about the Cross of Christ is not the message the world wants to hear today. But it's the message they need to hear! Let's keep preaching it!