How do we know that salvation is through preaching?

In 1 Corinthians 1:21, Paul notes that it pleased God through the foolishness of preaching to save those who believe. This demonstrates God's sovereign choice to utilize human communication of the gospel to effect salvation. It strips away any glory from human effort and positions the entirety of salvation in the hands of God. The essential point is that God's design is to work through preaching, and this aligns with His purpose to glorify Himself and His Son, Jesus Christ, in the salvation of His people.