Can materialists, atheists do moral reasoning as they claim? Former atheist and the late, Cambridge English Lit Professor, C.S. Lewis saw the flaw in their reasoning. It helped prompt his leaving atheism. In his book Mere Christianity he sees the blind spot as follows:
“The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. But the standard that measures two things is something different from either. You are, in fact, comparing them both with some Real Morality, admitting that there is such a thing as a Real Right, independent of what people think.” (13)
Ravi Zacharias puts it similarly: To have evil, you must good to measure evil by. And to have good, you need a Moral Law. And to have a Moral Law you must have a Law Giver.
So how can atheists seem so sure that they are able to know right from wrong? All people are created in the image of God. This package includes a conscience. Materialists borrow from God, from the metaphysical. The conscience is a wonderful grace of God. He gives it, not dirt, rocks, starlight or atmospheric gases.