Yeah, I'm not sure that objective morality's existence has anything to do with religion other than the fact that for some people to get other people to not do fucked up things they had to invent a god and impart morality on some people through that way. I entirely believe that it's just humans telling other humans how to act. Rules and social norms that govern how people live their lives and what they believe were, in my opinion, most likely invented to get people to act and behave pleasantly. As well as the fact that religion can/could explain things that are/weren't explainable.
It's also just as possible that there really is some higher being that created objective morality. But then, which higher being is correct? Or are they all the same... and people coming up with different organised systems are all just misinterpreting the spiritual feelings imparted to them? Are people that lived a good life according to multiple religious texts, including the "correct one" barred from heaven (or whatever) if they didn't believe in the "true religion."
And I don't think you or I or anybody can answer where objective morality stems from. We'll either find out when we die and it turns out some religious people were right and there's some sort of higher being that created everything. Or we won't ever know ever, we'll just die never knowing it. Anyone can believe whatever they want to believe, true or not. But to me it all just seems like a waste to believe in any of that or worry about any of that, when there's so much that's actually worth expending energy thinking about in life.