The challenge: ask your believing friends what they would do if it could be proven that their belief is totally unfounded... no god, no reward in heaven later

my guess: first answer:
"you'll never convince me that my god and heaven don't exist"

followup question:..."OK..just imagine what you would do if God and heaven did not exist"

probable answer:
"I'd eat, drink and do Mary...(or Steve, or even be Mary) over and over..cause tomorrow I die.

less likely answer...but still possible:answer:
I'd still try to be a good person"... so that others would b e good to me.

and there you (could) have the basis for the rational development of morality without the carrot of heaven or an angry god carrying a big stick

its the golden rule..which has, unfortunately, been interpreted differently in the past...