How is that phrase any sort of future tense?
It's not. It describes a condition already accomplished.
Consider the legal phrase -- "I, being of sound mind and body..."
That's not an assurance of some future disposition, but a current, ongoing condition already achieved.
I never said all references to justification are in the future tense. This one is in the present continuing tense. "I am being saved, I am being justified, I am being educated", all refer to a process that is ongoing and uncomplete. "I was justified" would refer to past tense. In "being of sound mind" "being" is not forming a passive voice, so it is a different grammatical pattern. It is nevertheless, likewise pointing to a present, rather than past, condition that may or may not obtain tomorrow.