What it means for God to be the ground or foundation of morality:
If we may assume (as I do) that God is the Universe, it is too complex a problem to analyze and solve without some process of simplification.
I would simplify it by focusing on one analogous aspect of God, one feature that is easily accessible to us inhabitants of Earth: the Sea.
The oceans cover approximately 70% of our planet's surface, and the total volume of salt water has been estimated to be about 1.332 billion cubic kilometers. It's way bigger and more physically powerful than the entire human species.
It is filled with both beauties and dangers. Without it we couldn't survive at all. Yet we typically don't think about it much, unless we see can see and experience it firsthand. Then its power and beauty easily captivates our attention.
Many of us are so attracted by it that we often decide to jump in and immerse ourselves in it, to surf its waves. We sense that there is a mysterious healing energy in the water, because we feel so indescribably good afterwards. We look forward to the next adventure. We plan to visit the beach often. We can't help but love the ocean because it seems to love us in some strange manner. We can even travel around the world on it in sailboats, it's so nice.
Usually all goes perfectly splendid, and we return home satisfied with our experience.
But there have been a few close calls, like the time we went to the beach for a swim and found hardly anyone else was there that day. There was a rumor, something about a riptide. But we weren't listening, because it took two hours just to get ready and travel to the beach, and we decided we weren't going to waste the trip. What would it hurt to take a little dip as long as we're here anyway?
But we were mistaken. I was enjoying myself so much and feeling so brave I went out a little too far. Oops, immediately I realized I shouldn't have done that; but it was already too late by that time. The rip current caught me and pulled me under and swept me further out to sea than I had intended to go. It savagely pounded my body into the sandy muck below, then viciously swooped me sideways and backwards into deep water. I felt like a powerless little rag doll. My trusted friend, the ocean, was toying with me. How shockingly unfair!
I thought it was over for me. Why had I been so foolish, so rebellious? Why didn't I pay attention, and listen to the warnings? I was too young to die now! The aggressive fighter in me struggled with the violent forces of the current. I fought relentlessly as with highway robbers, for my very life. I was trying to make a bee-line for the shore, but it was futile. The tide overpowered me and dragged me along like a clump of seaweed.
There was no way for me to conquer the ocean. It had me. The same entity that always gave so much was now holding me accountable. I shouldn't have gone for a swim that day. I should have been content enough with just getting my feet wet.
That was my first mistake. The second was trying to resist the ocean's stupendous power. I thought the ocean loved me! Why does it hate me now? This is so confusing! "Please forgive me and let me live to see another good day at the beach!!"
There is a "naturalistic" way of grounding or founding morality.
Before losing all hope, before giving up and accepting my tragic fate, I decided to try again. Fighting and struggling didn't help at all, I only ended up further out to sea. At that rate, I'd soon be lost forever and they might not even find my body after the sharks got through with it. Then I suddenly realized it was because I was swimming against the current. I had wasted so much energy trying to achieve the impossible. I was no match for the sea.
It has something to do with truth.
This time, I would lay aside my fear of the ocean. Instead of fighting it, I would try to work with it, to understand it. The ocean is wild but it doesn't hate me. I have always loved the ocean and felt its love for me. It has done so much for me and I am grateful, even if I can't have my way this time.
I made a big mistake but I'm not dead yet. Battered and weak, yes. But there is still some hope. If I conserve my energy and relax a little, maybe I can wait for the right moments to steer a little closer to the shore. Making a beeline is out of the question, I see that now. I will have to patiently roll with the flow until it is done with me. It will sweep me sideways for miles, until I'm no longer anywhere near where I started, where I feel I'm 'supposed' to be. But that is no longer important. I'll cross that bridge when I get there. I will find my way back home. My priority now is just to survive my embarrassing lack of good judgment.
God (somehow) makes moral propositions objectively true.
Much later, my feet finally made triumphant contact with the shifting sand. By the time I could finally extricate myself from the foamy, swishing wavelets, I was faint with exhaustion and my loved ones were still racing around frantically searching for me. They were crying, too; and I felt guilty for putting them through all this, despite all that I'd gone through myself.
They gathered me into their arms and took me home to a warm bed (I didn't have much of an appetite at that point). I was so tired, but they made me promise before they would allow me to take a nap. They said, "Don't ever do that again. We almost lost you."
And I answered, "Don't worry, I learned my lesson: Respect the power of the ocean. Don't disobey its laws. When there is a riptide, don't go for a swim; just be content to get my feet wet, until the weather changes for the better."
"Is that all?"
"No, there's more. My understanding of and cooperation with the ocean is what saved my life. Had I continued fighting, I would have lost that battle. Such a strange paradox, so counter-intuitive."
"You might want to stay out of the water from now on, just to be safe. We don't need any more close calls."
"No, I can't do that. I love the sea too much to stay away. After I've recovered from this trauma, I will look forward to our next visit. But I will be much wiser from now on."
"You're so stubborn! You nearly died! And next time it could be a shark or something else!"
"I know, but it's a calculated and reasoned risk that I'm willing to take. I learned my lesson: I know I have to respect the sea. Today I became personally acquainted with it.
"Stop it, the ocean is not a person."
"Of course it's not a person. Yet it's important that we learn how to properly relate to it."
The Universe is not a human being; but it is God, and we must learn how to properly relate to it.