"Being spiritual has nothing to do with what you believe and everything to do with your state of consciousness." - Eckhart Tolle
Labelling something doesn't change it, it's the state of mind. No matter what Earthly label you attach to someone (Bad, good, evil, atheist, religious), that is all it is. Merely Labels.
The state of the person's conciousness and personal experience is what determines what happens when they die, and is the result of the belief systems throughout life.
If they're atheist, but know about the afterlife, jesus, buddha (Most people have) then when they die they might be a bit confused at first, and slowly they would realize that they are dead and so they'll start asking themselves, maybe I was wrong? Then whatever belief system they change to this is the one they will experience in Death.
Remember death is just as much an illusion as "real life", and that pure consciousness is ultimately what we are.