Because atheists do not acknowledge objective moral law - one that applies to every human, Romans 2:12-16, particularly verses 14-15, would apply:
12 For all those who sinned without law will also perish without law; but all those who sinned under law will be judged by law. 13 For the hearers of law are not the ones righteous before God, but the doers of law will be declared righteous.
14 For when people of the nations, who do not have law, do by nature the things of the law, these people, although not having law, are a law to themselves. 15 They are the very ones who demonstrate the matter of the law to be written in their hearts, while their conscience is bearing witness with them, and by their own thoughts they are being accused or even excused.
16 This will take place in the day when God through Christ Jesus judges the secret things of mankind, according to the good news I declare.
Atheism cannot explain morality at all, because atheism does not have an objective definition of morality.
This is not to say that atheists have no morals. However, here is the difference:
An atheist can have morals or not have morals, but neither stance would deny their atheism.
A person who, say, believes in the teachings of Jesus Christ, automatically knows that if they do wrong, it is displeasing to God, and since he or she loves God, that knowledge provides a strong incentive to change their course and do right. Indeed, the definition of what is right and wrong is imbedded in the teaching of Christ.
The "lists" of immoral behavior that many atheistic books take out of context are designed to malign God's sense of morality, by attributing human morality to God. All these lists are an unspoken confirmation that humans need the God of Justice, who alone is real, among all other gods. These lists demonstrate that immoral behavior comes from human imperfection, however, when the Bible records it, this does not mean that God approves of it.
Also, if God does punish someone who does wickedness, just because humans don't agree with what wickedness means, does not mean that God is wrong and humans, especially those who were not there, are right.
The truth is that humans, despite their assertions to the contrary, are influenced by a multitude of things in life that could make them do wrong, and even justify their wrongdoing. So even though God says we shouldn't kill, humans have, and will, justify killing by saying that they came to them to teach them the gospel of Christ, or, some atheistic philosophy, and then upon their rejection, they kill them. Jesus never taught this, but it is their justification.
The Bible presents itself as a guide to living, not a textbook or an entertainment. And it is certainly not a specimen to evaluate by humans according to their view of right and wrong. Multitudes have lived, married, raised children and gave of themselves to the less fortunate because of what they have read there. I personally have seen generations of children grow up to be loving productive adults.
But an atheist can have morals or not and still be an atheist. The so-called "reason" and "logic" in this stance fails miserably in the face of greed,,passion, jealousy,pride, ambition, pain, fear - I could go on and on, and all of these at one time or another will induce humans to do stupid, self-destructive, and ILLOGICAL things. Atheists say that their choices are completely based on reasoning and science and logic, but humans have a tendency to lie to themselves, too, and their choices too frequently turn out to be based on any one of these kinds of things.
People who truly accept the existence of a Creator also know that life is sacred, and a gift from someone who loves them. This would certainly give them an incentive not to waste it, destroy it, or give it back.
For example, slavery, as it was practiced in the US, was entirely inhumane and immoral, and in history, one can notice the infiltration of the division of different races of humans who are evaluated differently. In reality, there is only one race, the HUMAN race. Also, Social Darwinism provided many heartless leaders with the pseudo-scientific justification for practicing some of the worst atrocities in human history.
On the other hand, it was people who espoused and lived Christian principles who took the lead in opposing these things, and it was they who established that all humans are deserving of their humanity, because they believed that all humans came from the same original pair.
Slavery, as described in the Bible, was not the same. It wasn't pleasant, but in most cases, such as for the ones who sold themselves into slavery, or in the case of war and devastation of their land, it was either that or starvation.
Acceptance of the existence of an absolute example - an intelligent, purposeful Creator, can move one to believe that all humans are brothers and sisters. Acceptance of an absolute unimpeachable moral code, one which includes mercy and compassion, but which no one can influence with bribery or threats, can move one to believe that we are obligated to treat one another with love and respect.
My view of the Bible is as a result of what I see in people who sincerely try to follow its teaching -and how the Bible has changed their lives. My view is also influenced by watching people who malign it and take it out of context, and who live according to the worst of this world. I see a sermon, clearly.
The Bible presents itself as a guide to living,not a textbook or an entertainment. Multitudes have lived, married, raised children and gave of themselves to the less fortunate because of what they have read there. I personally have seen generations of children grow up to be loving productive adults. Not PERFECT people, but people who try to keep God's reminders in their heart, and who try to keep their promise to their Creator, Almighty God, because they have come to love him. Psalm 119:97-99
The God of the Bible sent his Son to provide a visual example, and to provide a way of salvation, eventually, from the sinful tendencies with which we still find ourselves. This makes moral sin something to avoid, which, again, atheism in its basic philosophy does not completely condemn.
Atheists sometimes point to people who are atheists and who are moral in their estimation. It should also be noted that atheism does not make them moral. There are atheists who are very amoral but who are still atheists. If a Christian is amoral, they are acting against the teaching that they apparently accepted at some point. Jesus himself said repeatedly that if someone comes to him saying "Lord, Lord" but does not do the will of his Father, he will reject them. Matthew 7:21; Luke 6:46
So we have an often very effective incentive to do better morally in our lives, by our acceptance and love for our Creator, and by the example set by Jesus Christ.