system
Given the following post, respond as if you're commenting on Reddit:
human
(How) Can morals exist without religion?

I've recently left Christianity, and this is the main question I'm grappling with. 
Take what are, at least in my opinion, the two basic "moral" standards most people hold: the golden rule and justice/fairness. From a religious perspective, actions in line with these principles are objectively good because they please god. But without religion, are there any real grounds for saying that a just act is somehow better than an unjust one? Or that serving others is better than serving self? Couldn't these principles be reduced to modes of behavior people tend to like?
I am familiar with plato's argument in the republic that justice is beneficial to the individual and society as a whole, but I don't really find it all that convincing (as grounds for cleaning that justice is "good" and injustice is "bad"), so I am more interested in theories that don't rely on self-benefit (Or perhaps something more convincing than plato).
gpt
Most philosophers believe that even if God exists, morality would not depend on God, because either morality is just what god wants, in which case it is arbitrary and so there is no reason to follow it, or God knows moral truth/the correct moral standard, so it exists external to him and we could presumably find it.  The question of what this standard is though, and how it is justified, is the fundamental question of ethics, so there aren’t easy answers.  Some philosophers appeal to intuition, or intuition refined by formulating principles by abstracting from (some of) our individual moral intuitions and then testing those same  principles against (other) intuitions, ultimately bringing them into harmony   (this is known as “reflective equilibrium”).  Some (mostly Kantians) argue that morality is entailed by the standards of “practical reason”, or reasoning about our actions.  Some try to analyze/reconstruct the logic of moral language  or moral argumentation and develop moral theories based on that.  There are many different theories.