Johnston took no objection and in fact adopted the idea to make an evidence based case for the existence of God known as the argument from morality. Kudos to him for being prepared to change his mind.
In laying out the argument, Johnston made some common claims that I hear apologists make and which he explained like this: we can’t have universal moral truths without a transcendent moral truth giver. That is, without God, we have a philosophical problem in that moral relativism must win the day because there are no moral truths – just mere human opinion. In that case, whatever moral custom prevails must be a result of popularity or of “might being right”, which clearly isn’t right. We need a God “WHO SEZ” what’s right for everybody.
But I wonder, how does God solve this problem? I mean, who says that what God says is morally true? Isn’t what God says just God’s subjective opinion of what’s moral? Isn’t it just that he is the mightiest? Doesn’t this "solution" just push these problems back a step and project them onto God?
Imagine that in my basement, I can genetically engineer human beings with an IQ of around 65, and I design a few to think that rape is good. When they wonder why that is, I tell them that as their maker, I have made it so and made them so. They have a transcendent source of moral principles, which is just what Johnston ordered to avoid relativism. Is this an acceptable state of affairs absent the philosophical problem alluded to earlier?
When God commands genocide in the Bible (1 Samuel 15:3), or a flood that drowns virtually all men, women, and children (Genesis 6:13), or provides instructions to guide chattel slavery of non-Hebrews (Leviticus 25:44-46), is he endorsing something good? Either way, whoever claims that the Judeo-Christian God grounds moral truth seems to still have a big philosophical problem on his hands.
A transcendent source provides a kind of reason to impose or enforce moral rules on others, but I just don’t see how that source makes those rules really moral and objectively true. And so we arrive at the famous Euthyphro dilemma, attributed to Plato:
"Is what is morally good commanded by God because it is morally good, or is it morally good because it is commanded by God?"If one accepts the first horn of the dilemma, then God doesn't ground moral truths. If one accepts the second horn, then there aren’t moral truths – just what "God almighty SEZ”.
The rebuttal that moral truths stem not from God’s opinion, but his nature, also fails because it just pushes the dilemma back yet another step:
"Is what is morally good so because it is a part of God’s nature, or is what is morally good a part of God’s nature because it is morally good?"
Nobody knows what God says. All we ever hear is what different people say that God says, and since there's no way to know, on divine command theory, we end up with thousands of different religious sects all claiming that God says different things. When what God has apparently said in the past is criticized, believers often respond that his previous moral advice was contextual. If that's not moral relativism, then I don't know what is.
If one wants a universal approach to moral questions, something that everybody can appeal to, the leading candidate has to be reason. There are reasons why certain behavior is morally good or bad, and they have to do with whether the behavior increases the well-being vs suffering of conscious creatures.
In the debate below, I think that Johnston's concerns are voiced by the infamous Dr. William Lane Craig. On the opposite side, Dr. Shelly Kagan outlines a framework for moral considerations based on reasons that all rational people would and ought to adhere to known as contractarianism. There are others; secular moral philosophers have been keeping themselves busy for hundreds of years doing this stuff, no appeal to God necessary. I encourage you to watch the debate if this topic interests you because it ends with a cross examination that is very friendly and polite and really turns into more of a conversation between these 2 philosophers who are experts at articulating their respective positions. Your best bet at seeing who's most able to defend a position comes not when debaters are giving prepared statements and anticipated rebuttals or even when dealing with audience questions. It's when the debaters have to answer each others questions.
I’m quite partial to Sam Harris’ moral landscape as a way of thinking about an objective moral framework. It shares many similarities with Alonzo Fyfe’s “desire utilitarianism”, which I actually think is the best moral theory out there, but Harris’ work is more available to the masses.
As it is with ontology, so it is with morality: so long as we can draw from reason and evidence, we have an ongoing conversation that gets us closer and closer to the truth. As Martin Luther King (almost) said, “the arc of the moral universe is long but (with religious conservatives often kicking and screaming) it bends toward justice”. In the absence of reason and evidence, divine command theory (the name of the moral framework Johnston is endorsing) requires that we consider something very dangerous ... something other than the well-being and suffering of conscious creatures, namely, the conversation-stopping dictates of mysterious and almighty Gods. This seems much more worrisome to me than the problems Johnston identified.
In a great speech, American President, Barrack Obama, himself, a Christian, seems to agree:
"Democracy demands that the religiously motivated translate their concerns into universal, rather than religion-specific, values. It requires that their proposals be subject to argument, and amenable to reason. I may be opposed to abortion for religious reasons, but if I seek to pass a law banning the practice, I cannot simply point to the teachings of my church or evoke God's will. I have to explain why abortion violates some principle that is accessible to people of all faiths, including those with no faith at all."