Cognitive dissonance. I’ve seen some atheists define it as doublethink, compartmentalization, or any attempt to dismiss conflict or rationalize it away, but cognitive dissonance is supposed to describe the internal discomfort people feel when they realize they “hold two or more contradictory beliefs, ideas, or values at the same time.” It’s stressful. That means they are not happy, beneath the smugness.
That’s something that can be worked with — sometimes. Help them argue themselves out of it.