“That obviously covers those with whom neither were Christians before being married, but then one became Christian.”
1) Define Christian
2) Is it in the slightest bit possible that depending on your election belief
a) The person was never regenerate but proclaimed they were a christian
b) The person left the faith
There are many things we can “judge” (discern) as Christians, but we cannot judge if someone has lost their salvation, nor can we pronounce punishments as though they have.
We are allowed to punish bad behavior and to excommunicate people who appear to have crossed such lines, though.
Regardless, our vow to God remains if we were Christian.