Well, it is a mystery for Christians, and I'm trying to understand the differences between the two camps. From my perspective its not even an issue.
As you've already nearly surmised, it speaks to whether man is truly a free agent or not. Which speaks to philosophies of how exactly, or how much, input man has in his salvation.
Obviously, a person who believes that God is determining everything can feel only grateful for having been "saved" (or so they think). Where a person who recognizes in the act of Creation a delegating of power and authority to man to make his own outcome, would be more inclined to realize that our actions help to determine our fate.
There's a lot more, as well, but that's the basics. Calvinism and "Faith only" go together because they must.
SD