The great Greek philosopher Plato once posed a very profound dilemma about the nature of "good" and virtuous action that atheists and other anti-religious minded people have hailed as one of the most devastating intellectual blows to theism. Specifically, it seeks to destroy the claim that apart from God there can be no absolute standard for good and evil.
In his Dialogues, Plato tells a story where Socrates comes into contact with Euthyphro, a man who is on his way to the courthouse to testify against his own father on an account of murder. Socrates is astonished by this and asks Euthyphro why he is doing such a thing. Euthyphro responds that it is the pious (right and good) thing to do.
Socrates then asks the question: "Do the gods wish for what is good, because it is good; or is what is good, good, because the gods wish for it?"
This is indeed a provocative question and I've heard it before but in my view and understanding, it is not anywhere near as devastating or insurmountable as atheists might like to think.
As a Christian, I will answer this from the Christian perspective specifically, meaning that "the gods" will refer to the one triune God of the Bible.
Does God wish for what is good, because it is good; or is what is good, good, because God wishes for it? In speaking of the theistic-Christian concept of God, I would have to say that the answer is the latter. In other words, good is good because God wishes it. The latter option, that God wishes for good because it is good, cannot be true because if it was, God would not be needed to define good.
But wait, the atheist argues, then if the latter is true then God can say anything is good, even killing people or children. Yes, that's exactly right. God is the sole determiner and definer of what is good and what is evil. But before I get to that point, I will more directly address the dilemma at hand. For the atheist who quibbles that God could or would consider something most people would consider evil such as murdering children as being good, they are making an irrelevant and self-defeating claim. I would then ask them, "why do you think murdering children is wrong? On what grounds and on what basis do you believe murdering children is wrong?" The most likely and truthful answer they could give is that their own conscience tells them it is wrong. At which point I would ask "Where does your conscience come from and why do you have this innate sense of moral judgment to begin with?" The two most reasonable answers would be that either it is an internal mechanism produced by natural evolution, or it is an a priori awareness of divine moral law that was instilled by the Law Giver, God, who made mankind in His own image and made them moral beings. In this case, I am obviously claiming that the latter is true.
If the latter is true, then the one who criticizes and condemns God's ability to define goodness is doing so with the very same standards God gave them in the first place. Thus, their argument becomes circular and self-defeating. In effect they are saying "God can't define good because then God can say things that are evil are good, yet the very things I consider to be evil, I consider evil because God has said they are evil to begin with." Why is murdering children wrong? Because God has established that law. Why do most people believe murdering children is wrong? Because that moral law which governs us is instilled into our being, giving us awareness of what good should and shouldn't be.
Why does God have the right to define good? Because He IS good. He embodies the essence of good and good is His very nature. Because of His omnipotence and goodness, He created the universe and has authority over all of it. For every law there is a law giver. Just as gravity is a law that governs the workings of the physical universe, God's moral law so governs and judges the actions of human beings. God is also just and immutable. He does not change. His nature does not change and His principles and standards do not change. That means God is not going to suddenly change His mind someday and say that something that is evil is actually good. Jesus made the same challenge about people's notions of good when he was called a "good teacher." ‘Why do you call me good? No one is good except God alone.’ Mark 10:17-18 In other words, Jesus was pointing out that the only true definition and standard for all good is God Himself.
And like I have said before, God is necessary for morality to be objective. Humans possess a moral conscience and an awareness of divine moral law, but at the same time, because we are sinful and corrupt in our very nature, our consciences and sense of right and wrong are also corrupted. This is the reason why people will widely disagree on varying issues. With such common confusion and discrepancy among so many moral issues, it would seem that morality cannot exist in any meaningful sense apart from God. For if that were the case that good stood on its own apart from God, then it would seem more probable that people be in further agreement on what is good but they are not. C.S. Lewis made the comparison between flawed human perceived morality and true objective morality like this: "The Divine 'goodness' differs from ours, but it is not sheerly different; it differs from ours not as white from black but as a perfect circle from a child's first attempt to draw a wheel. But when the child has learned to draw, it will know that the circle it then makes is what it was trying to make from the very beginning." Humans have a basic grasp of God's goodness, but naturally it falls far shy of His definitions and therefore we must reform and align our morals to His absolute and objective standard.
If God did not exist, then there would be no basis to say that human beings are corrupt in nature. If God exists, then there is an objective and immutable standard for good and evil that applies to every single person and does not change based on any one person's opinion. Without God's existence, we have no objective means or absolute standard by which to judge what is good and what is not.
Further, if we are to believe that the universe is a purely natural atheistic one, then the concept of good and evil do not exist. Relative morality has no value and no more effectiveness than a relative law does. The only absolute truth in a Darwinian world is survival of the fittest. The strong survive and the weak die. Morality has little do with survival and in fact, it often impedes survival because morality dictates that you put the concern of others before yourself. Survival is about looking after yourself and your own best interests. Because of this reason, it is illogical to believe that the concept and awareness of morality was something that merely evolved or was produced naturally, especially when you consider the fact that only one species on this planet happens to be motivated and controlled by moral values.
To sum it up, I don't see what the problem is with the idea of a God who is intrinsically just and good defining what good and right action is. I know that some would say that my reasoning is too simplistic. I say the most powerful logic is simple logic.
It should also be noted that the appeal to Plato by atheists is somewhat ironic considering Plato was actually very conservative-minded. He was deeply concerned with the moral depravity he saw around him in his time and culture and he was also a big believer in the metaphysical. He believed that reality was better represented by the spiritual or immaterial realm than the physical one and also rejected the pagan Greek gods and adopted what could be considered quasi-monothesitic beliefs.