Why does it matter whether we understand what God accomplished at the cross for us?
It matters because God already poured out all His mercy at the cross; He does not have to do it again. Understanding this lets a believer appropriate that mercy by faith, saying, 'Yes, Lord, I believe it; it is mine,' instead of asking God to repeat what He has already accomplished.
Les teaches that a sinner's plea for God to be merciful is not appropriate today, because God has already poured out His mercy at the cross. To ask Him to pour it out again is a double take, as if God must do it again; He does not, for He has already poured out all the mercy of a sovereign God. The believer's part now is to appropriate that mercy by faith: 'Yes, Lord, I know that Your mercy was poured out. Now I believe it. I appropriate it. It's mine.' Les says this makes all the difference in the world if we do not understand what God has already accomplished at the cross on our behalf. He connects this to Romans 9, where God told Moses He will have compassion and mercy on whom He will, because He is sovereign and the final authority; verse 16 says it is not of him that willeth or runneth, but of God who shows mercy.