The GDR quote seems to be saying something different from the thread title. It says that the absence of God leaves us with no frame of reference for understanding morality, not that its existence proves God. (I didn't see the discussion this branches off from. The quote doesn't mention God, but He seems strongly implied.)
But does morality have any actual reality, or is it all relative, like the familiar war conundrum where killing a man from an enemy nation is moral, while killing a man from your own nation is immoral. Or consider the bombardier conundrum where dropping bombs on innocent civilians in a city of an enemy nation is moral but killing a captured civilian of that same nation is immoral.
And even if morality is something real, is God really a frame of reference for it? Was the OT God moral? Or is it that, to paraphrase Nixon, if God does it it's moral?