Why did God allow Adam to sin?
Adam's sin was permitted because it was in the sovereign workings of God that humanity be left with an alternative. Otherwise there would be no choice or exercise of the will; people would be like robots. God created beings able to accept or reject His offer of salvation.
Les teaches that it was in the sovereign workings of God that man should be left with that alternative. Otherwise there would be no choice and no exercise of the will. If Satan had not been permitted to come into the garden and sin had not entered, the human race would have been without choice, like robots. But humans are not robots; they are created beings given a choice to reject or accept God's tremendous offer of salvation. He also notes that everyone is a sinner because it started with Adam, and death passed upon all men.