The moral argument for God comes in its classic form from C.S. Lewis’s Mere Christianity. Dr. Sproul liked to trace it back to Immanuel Kant, who rejected the Classical Apologetic view and inserted this one instead. I would recommend Lewis, who didn’t blatantly contradict Romans 1 like Kant did.
Lewis pointed out that humans everywhere have a moral compass. Every culture has values that it calls good and vices that it calls evil. Everyone, in other words, has a conscience with a standard of good and evil. That is, at least as far as everyone else goes. We all expect others to behave with certain propriety towards us even if we don’t feel like reciprocating. Even Hitler felt he had been badly wronged when Himmler tried to desert to the Allies. To feel wronged like that, you need a rule of good and evil.
Lewis agreed that there were variations between what behaviors cultures would accept or not. He did not see this, as some do, as indicating that there is no fundamental moral standard, since if you drill down far enough you eventually get to some common ground. His example was that, in the West, we mandate monogamy, whereas other societies have no problem with polygamy. He couldn’t name a culture, however, where you could sleep with just anyone you wanted without moral censure of some kind.
Many people believe good and evil are just concepts built into the human race as a survival mechanism, but Lewis had answers for that too. He said that when someone wrongs us by accident, we aren’t as angry with them as we are with someone who tries to wrong us on purpose and fails. Didn’t the accidental person do more real damage? Or consider a soldier who falls on a grenade to save his buddies. I don’t think anyone with any heart would consider that anything but a good action, but that person has just terminated all possibility of passing on his genes to the next generation, and that’s what the survival mechanism in animals is all about. They’re so craven about risking the slightest injury that a handful of wolves can put a herd of buffalo, each many times their size and armed with powerful hooves and sharp horns, into panicked flight. When humans desert their comrades, however, we view that as a morally reprehensible action. That’s not the way the survival mechanism works in nature.
Lewis and Kant pointed out that, if this moral standard is to mean anything, there have to be rewards for good behavior and punishments for bad. We know that life tends to do that, but it doesn’t always. Hitler went one country too far when he declared war on us and wound up having to shoot himself, but Stalin and Mao, who actually killed more people, died with their supreme power over Russia and China still theirs to enjoy right up to the end. Evidently, in order for us to say they were wrong when they in the end got what they wanted, there’d have to be some kind of punishment for all the evil they’d done after their deaths.
That would require there to be a judge over the human race. He needs to be omniscient so that he knows what we do, he needs to be omnipotent so he can enforce his judgment, he needs to be incorruptible so he can’t be bribed, and he needs to have created the human race so he has the authority to judge them. In other words, you need the Christian God. Indeed, without a perfect God to set the standard for good and evil, you can’t have a standard at all. No human has the authority or infallibility on earth to lay down perfect rules for how humans should behave. Without God, there’s no reason someone should be a Martin Luther King rather than a Joseph Stalin.