2 Nephi 30:17

Dave, In my opinion I think the scripture is referring to how in this world there is nothing that cannot be fixed. Even sealing on Earth can be loosed if it absolutely needed to be. The darkness and light part is talking about how there is no evil that cannot be overcome by goodness. Satan can never have power over the light of God. In the next verse it says how Satan shall no longer have power over the Children of Men. Basically, Satan can only have as much control as we give him. I think that is what the scripture is referring to, but I could be wrong. Just my interpretation.