Why can't man work for his salvation?
Salvation cannot be earned by human effort because it requires the sovereign power of Almighty God. Man possesses only physical strength, while salvation concerns the spiritual, which only God can bring about. The preaching of the cross is the power of God to those who are saved, not something attained by works.
Les explains that salvation has to be an exercise of the sovereign power of an Almighty God; that 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. Christ has paid the sin debt, suffered our punishment, and opened the windows of heaven if we will just believe it, even though it happened 2,000 years ago. The preaching of the cross is foolishness to those who perish, but to those who are saved it is the power of God. Les also notes that the Jews require a sign and the Greeks seek wisdom, but Paul preached Christ crucified—a stumbling block to the Jews and foolishness to the Greeks—yet to the called, Christ is the power and wisdom of God.