Why is preaching Christ and Him crucified essential to the gospel?

In 1 Corinthians 2:2, Paul emphasizes that he resolved to know nothing while among the Corinthians except Jesus Christ and Him crucified. This proclamation is the core of the Christian faith, as it reveals God's plan for redemption through Jesus' sacrificial death. The gospel message hinges on recognizing humanity's sinful condition and God's saving grace through Jesus. If any preaching deviates from this focus, it fosters an uncertain sound, detracting from the power and effectiveness of God's truth to save and transform lives.