No moral issue can be debated, in any meaningful sense, without reference to God. Take God out of the picture and all we are left with is the accidental product of time and chance acting on matter.
Doug Wilson has argued that once we remove God from the debate, “then the difference between your thoughts and mine correspond to the difference between shaking up a bottle of Mountain Dew and a bottle of Dr Pepper.” You have no basis for arguing that your thoughts are nearer to the truth than anyone else’s thoughts. You fizz one way, they fizz another. This means you don’t hold to your view because it is true, but because of accidental chemical reactions taking place in your brain.
Ideas of morality, justice, and logic are all mere sensations. Wilson suggests, “this means that we have no reason for assigning ‘truth’ or ‘falsity’ to the chemical fizz we call reasoning, or ‘right’ and ‘wrong’ to the irrational reaction we call morality.”
C.S. Lewis made a similar observation, when he pointed out that if there is no God, then nobody designed our brains for the purpose of thinking. “It is merely that when the atoms inside my skull happen, for physical or chemical reasons, to arrange themselves in a certain way, this gives me, as a by-product, the sensation I call thought. But if so, how can I trust my own thinking to be true? It’s like upsetting a milk jug and hoping that the way it splashes itself will give you a map of London.”
Without God we have no reason for trusting our own thoughts. “Unless I believe in God,” Lewis concluded, “I cannot believe in thought: so I can never use thought to disbelieve in God.”