There’s been a lot of work in the past couple hundred years on reasons to behave morally that don’t require a hierarchy of supreme power.
Game theory, capitalism, free markets, bottom-up economics, wisdom of the masses, democracy, emergent behaviors – these all relate the idea that when people behave well all benefit.
In fact, I’d argue that leaving God out of it makes for a stronger system. If you take the original idea that God grants rights to all people and we then give up certain of those rights to the Government in return for protection of the others, the only thing that keeps the Government from turning tyrannical is the existence of God. Stalin and Mao seemed to understand this.
If instead, you realize that it takes millions of people making smart decisions to keep our society working smoothly then you have to maximize the freedom of each of those actors to get the best possible outcome. Each imposition on those actors brutishly eliminates a possible beneficial outcome in an incomprehensibly complex system, so to the extent that such restrictions aren’t essential to the defense of rights required by us of our government, they only act to society’s detriment.
This model still strongly supports the free exercise of religion but does not require it. It also recognizes the value of each and every human being as integral to a society, while concomitantly being their own sovereign masters, properly casting a government’s role as the servant of society and mechanism to protect the weak. And by putting humans explicitly at the top (bottom?), it reminds those humans that they have to be responsible and make the right decisions, nobody else is granted a higher role.
Or, y’know, like Jesus said, “be nice to each other.”