Back to the original question.
If Zombie Christian = serving God but heartless,
then what does that really mean?
We all have peaks and valleys in our spiritual walk.
Sometimes we can go through periods where we even feel cold.
That's probably what you meant by "zombie Christian."
But if someone has really deep, serious, long-term heart problems,
it's natural to wonder if the person is even saved.
Only God knows.
But usually time will give us the answer.
Whatever someone is, or is not, will always become apparent in time.