The first phrase, “but this cannot be,” refers to the previous verse where Alma declares that should the unclean come before God, that we will desire to hide from his presence. That cannot be: “we must come forth and stand before him in his glory.” The final judgment places us, and all that we have become, before God and his righteousness. At that final time, a division between good and evil will occur. If we have not become good, we cannot withstand his presence. In shame, the unjust enter the reward of the wicked.
The opposite is also true. Those who have repented through the power of the Messiah’s name, and have lived according to the true way, shall be judged good. They will not be ashamed to be in the presence of the Lord.