Why is the preaching of the cross foolishness to some and the power of God to others?
The preaching of the cross is foolishness to those who are perishing because they rely on human wisdom or signs, but it is the power of God to those who are saved, since salvation requires God's sovereign power and cannot be earned by man's physical strength.
Les explains that the preaching of the cross is to them that perish foolishness, but unto us who are saved it is the power of God. Salvation has to be an exercise of the sovereign power of an Almighty God, which is why man cannot work for his salvation. All man has is physical strength, but only God can bring about the salvation of the spiritual. The Jews require a sign and the Greeks seek after wisdom, but Paul says they would not bother with either. Instead, they preached Christ crucified: unto the Jews a stumbling block, and unto the Greeks foolishness. Yet unto them who are called, both Jews and Greeks, Christ is the power of God and the wisdom of God.