If you have not read Stories Versus Truth, you may want to read that before reading this.

This particular question assumes moral superiority over God, doesn't it? See also Why think that God should define goodness?

Yet, if Atheism (or many other religious views) were true, how could you define good? Many Hindus, for example, openly admit that they believe that there is no good, evil, right, wrong, truth, or error. A consistent Atheist or Agnostic would have to say the same. For those who believe that all is random, there is no rational way to claim that one way of acting is righteous and another way of acting is unrighteous. Some of them do claim that there is good and evil, but they are being inconsistent and irrational by making such a claim while also claiming that everything happens by random chance (that is, that there is no God). For those who claim that there is a god (small g) but that nothing can be known about this god, they would have no way of discerning what is good or evil either.

When unbelievers are asked how they can determine good from evil, they may try to create a straw-man argument by answering a question that was not asked. The actual question asks for the logical basis of the unbeliever's concept of good and evil. Many times and unbeliever will insinuate that the question that was asked was, "Why can't unbelievers do anything good?" It is a fact that they really cannot do righteousness in the sense of God's definition of righteousness (study the diagram), but that is not what is being asked here. We are asking for their logical reasoning behind their particular definition of good. They must end up in an irrational argument. There is no other option for the unbeliever.
The reader may be interested in just exactly why things are as they are.  Read: How Everything Got So Messed Up and What God Is Doing About It  How Everything Got Messed Up and What God is Doing About It.  There are some additional links at the bottom of the page to help in understanding God's purpose.

It is a fact that the unbeliever must be irrational in defining good. They must be arbitrary, which is irrational. They have no absolute standard. If they did, what would it be?  Only the follower of Christ has an absolute standard that is both internally and externally consistent. That standard is that which is revealed by God Himself.  The standard is received by revelation in real time from the throne of God. Read "How Can We Know Anything?"

The fact is that, without the God of the Bible, there can be no objective good or evil. God defines good and evil. The purpose of asking the unbeliever to test his or her logical basis for judging right and wrong is to help the unbeliever to realize that their question borrows a viewpoint that can only logically belong to a Christian.

Interestingly, logic cannot exist outside of the God of the Bible either. There is no reason for the law of noncontradiction, for instance, to exist in the worldview of the unbeliever. For the Christian, the law of noncontradiction is just a revelation of the nature of God. Many unbelieving religions actually reject the law of noncontradiction, which is illogical and can easily be shown to be wrong.

The reason those who follow Christ believe is not based on physical evidence or human argument, though the observable physical evidence supports God's existence and His Truth without the mental gymnastics that the Atheists and Evolutionists find themselves tangled in. It is based in the power of God. Faith comes by hearing and hearing by the word (utterance) of God. God speaks. God speaks to every person. Whoever will listen receives the faith that is of God. This is a supernatural belief that is not human-generated. It is a gift. That faith is the belief that what God has said is true. Before a person is born again, there is a history of hearing God's voice. No one can come to Christ unless the Father draws him or her. A person is born again the moment that he or she hears God's voice and receives faith to believe in Jesus as Savior from sin. In the same way, it is God who speaks and says that the Bible is the Word of God without error. When we hear His voice, then His faith comes, the supernatural belief that the Bible is the Word of God without error. Once God opens our eyes, we see that the Bible is the only source that is both internally and externally consistent. We cannot possibly believe unless we are having an active interaction with our Maker. Some Christians are not aware enough of their own experience to know this. That's why we have all the previously-mentioned reasons for believing the Bible as the Word of God. (some caveats about hearing God's Voice)

Interestingly, explanation given in the previous paragraph does not depend on circular reasoning or poor logic. It depends on God. This is the kind of faith that is what is really operating. It is based on revelation. Now, a naturalist will attack the whole idea of revelation claiming that revelation cannot be verified.

• Ungodly People Miss Out On What Is Good--Ungodly Christians Miss Out Too read
• Don't Fall Short of the Wonderful Plans, the Destiny that God Has Planned For You. read
• God's Purpose For Our Lives Is That We Become Like Jesus read