It’s fairly easy to smoke out a half-atheist. A half-atheist is someone who loudly claims that there is no God, but keeps borrowing and using ideas and concepts from the Christian Bible.
The half-atheist, for example, believes some things are right, and some things are wrong. He objects to mass genocide, say. He believes no one should do that, which means it is a universal rule for everyone. He believes mass genocide is always wrong, in every situation, making it authoritatively binding for all humans. He believes people who commit mass genocide are guilty, should be stopped by others, should face justice, and should seek forgiveness and restitution.
In other words, the half-atheist believes in some objective moral values. Some things are objectively wrong or right, which places an obligation on all people to avoid them or embrace them.
Now we’re actually very glad our half-atheist lives this way. It would be dark and chilling indeed to live with a man who thought mass genocide was an efficient way that nature culls out an over-supply of human beings. But the point is that his atheism did not give him objective moral values. He had to borrow them from a theistic worldview, which he claims he rejects.
An atheistic worldview usually embraces some form of naturalism. Naturalism believes the universe came to be through blind chance, life came about through accidental events, and life mutated through random selection. In this worldview, there is no right and wrong that binds all humans, any more than there could be rules for wolves, or commandments for sharks, or microbial etiquette. In a blind and dumb universe, there is no Lawgiver, so there are no laws. There is only the will to survive, and to eliminate those who compete with me for survival. You cannot say to any evolved creature, “You ought to…” There is no ought, where there is no Law. There is only what is.
But our half-atheist definitely believes some things are true, and some things are good. He might spend some time telling you that truth is person-relative, and morality and ethics are situational, but he’ll drop all that when you accuse him of butchering babies and eating them. His loud protestations will tell you he believes in truth, and his revulsion at the idea will tell you he believes in goodness.
What we want to ask our half-atheist friend is where he got these objective moral rules in a blind, material universe. Objective moral rules are universal, binding, authoritative, and personal. A naturalistic universe is impersonal, without authority, and therefore relativistic.
It is a theistic and, ultimately, biblical worldview that gives us a personal Creator, whose own nature is what we call “good”. Out of that nature has come rules and laws, found first in men’s consciences, and then in the written codes of Scripture. People with consciences, people with innate objections to certain behaviours, and cultures across the world with laws and rules and codes are exactly what we’d expect in a world made by a personal, moral God, who communicated something of Himself to human beings.
No, the presence of morality does not “prove” the triune God of the Bible, nor does it give us the Gospel. But since it coheres with a biblical view of humanity, it places the burden of proof back upon the atheist. We ask him, where does your morality come from? We Christians can easily explain it according to our worldview. But by what standard do you atheists say anything is “good” or “bad”? By what authority can you condemn any behaviour? By what code could you say, “I am a good person”? Your blind, random, amoral universe simply doesn’t care. So why should you?