One of the most and least popular Sunday school classes I ever taught involved a lot of mud. “Most” popular, I say, because the students always remembered it. “Least” popular, because their parents weren’t all that happy with the results.

Early on Sunday morning, I walked out onto the church lawn with a hose. This was one of those formal churches, where the girls where frilly dresses and the boys wore handsome suits. On the front lawn of the church that day, I made a mud puddle. And by “puddle,” I mean lake. When the students arrived, I told them to take off their shoes and splash through the mud. There was mud everywhere. There was mud on their dresses and mud on their suits, mud between their toes and mud in their hair. However, it was all a necessary baptism into the world of clear thinking.

“If there is no God,” I told them, “this is where you came from.” I observed that a sheerly material world could produce nothing but highly-evolved mud. We might have all kinds of fascinating, highly developed traits, like language, cognition, and the ability to scratch our tummies while rubbing our heads, but at the heart of things, we were simply mud. “And if this is all we are, there is no reason why you shouldn’t treat other people like you are treating this mud.”

For the students, it was a living object lesson that they would not forget. For their parents, who had to let these muddy children into their cars, well…sometimes the youth pastor just has to apologize.

But what we caught in our little parade across the lawn was a visual representation of the moral argument. If there is no God, then fundamentally, morality amounts to little more than socialization, peer pressure, and meaningless instincts. Though an atheist could construct a reasonable explanation for kin altruism, at the end of the day, the sense for any innate or objective moral value attributable to humanity is simply a misfiring of neurons. Evolution may have produced in us a sense for teamwork so that we might survive, but true, objective moral value is simply an illusion.

I find when I present this argument in abstract terms in an apologetics seminar that I offer to churches, there is often a rational pushback from Christians and skeptics alike. It seems like atheists should be able to hold onto morality, since so many of them seem to behave morally. But when I present it in terms of the mud puddle, the dire consequences of atheism and materialism become hard to refute.

I hope that more than a few church youth leaders read this post and go make mud puddles of their own. The moral argument is likely to be on the forefront of Christian apologetics in the coming decades. If we’re not confronting students with the stark alternatives to faith, they may never realize how important the choice is.

Rev. Dr. James W. Miller is the author of Hardwired: Finding the God You Already Know and the Pastor of Glenkirk Church in Los Angeles, CA.