I don't think you have made too grim of an assessment. It is basically saying that what you do doesn't matter, because you are already saved or damned as it is.
fate and destiny are often confused which bring up strange ideas like predestination to allow people who have no knowledge on the subject to have a jaded, yet, concrete clue of the idea.
Fate would be the better word to use in this scenario.
Imagine balancing a ruler in the middle horizontally
the ruler is literally everything. the beginning, the ending, you, me, and everything in-between.
The ruler is fate.
Destiny is where your hand is, it is right now.
Now is all that matters
However, fate can be thrown askew, convicted, or imagined.
Now, you ask about pre-destiny? Sounds lifeless, still, in the dark...demons
i suppose but the way i see it, material as a blank sheet of paper that hasn't been written on... ...where was i going with this oh, you labeled eternity as immaterial. sure, it may not be as concrete as you like to think but i think it can be
Do I paint too blacker a picture of predestination? What do you think?