One of the reasons that my morality hinges on my belief in god is that without god, I see humans as 'just another animal'. One of the reasons that I desire to be good to people is that I think they are special, because of god. I don't really care about the other animals that much.
do you spend your weekends beating puppies and drowning kittens? i can't see otherwise how humans being "just another animal" would change how you treat them? just because humans are just animals doesn't make them any less like you or any less deserving of the same deference you show yourself. love your neighbor as yourself doesn't need a god to enforce it. it's just a good thing to do.
Without god, and any real meaning to our existence, none of this shit really matters. Selfishness isn't 'wrong', it just isn't good for everyone. But that doesn't matter either OTOH, selfishness is good for me, so there is a benefit to it.
but selfishness
isn't good for you. it reduces your public standing and can result in criminal sanctions. it can seriously harm YOU. and who cares if this shit really matters? how you deal with those around you affects you and affects the world your children will live in.
I'd still care about my family and friends even without god's existence. I don't think my apathy could go that far.
then maybe your problem isn't that if you stopped believing in god you'd think there's less meaning, it's that you already think your responsibility only extends to those you are imediately in contact with. but jesus said that our "neighbors" are anyone in the world who needs our help. your responsibility with or without god is to everyone. everyone.