For Luther, from what I could understand in his argument, his issue of certainty would be less stressful to me as an early 16th century reader, because it seems less worrisome. As Luther said in Part 2, Chapter 5, "both [my] creation and preservation come to pass by the sole will of the omnipotent power and goodness of God." If I'm a common villager living in the 16th century, I assume there is plenty of housework and farming and walking and surviving to be done. I as a hardworking commoner do not have the time or resources to dedicate to become a well versed devout Christian, so leaving it all up to God's omnipotence and grace sounds much more convenient and simple.
Certainty to Christians is a bit different, I think. That's the beauty of Christianity: we don't need to be certain in everything because we are certain that God will help us through whatever comes our way. Certainty in our beliefs allows us the confidence to be uncertain in everything else. Life is unpredictable, so it's impossible to be certain about temporal things. God is the only constant in a Christian's life.This entire idea of certainty relates to what was happening with the Protestant Reformation. All the sudden, there were plenty of new beliefs, sects and theories on how to know what was certain--which one was true. Luther, Erasmus and others all had their own ideas of how we could know what God wanted for us. How grateful I am that Joseph Smith decided to go directly to the source and ask God Himself what he should do. God was Joseph's only certainty.