For those who don't understand, I'll use Dante's Inferno as an example
The lustful are in the second circle of hell, near the top. Dante is seen even being sympathetic towards them; as lust is the most universal human weakness. Our Lady of Fatima stated that, "the sins which cause most souls to go to hell are the sins of the flesh." It's such a common problem that most people KNOW it's already an issue. Even an atheist can see that
Atheism on the other hand, while not explicitly mentioned in the Inferno (due to it not really being a thing during medieval times) would most likely be in the sixth circle with the Epicureans (some of which believed in the death of the body also equaled death of the soul). These are sins of the higher will: atheism is a rejection of the highest faculty.
Augustine and Aquinas would say the intellect is ordered toward Truth; and God is Truth in its fullness. To refuse that consciously is a more total rebellion than being dragged down by the body
So the irony holds: the sin that fills Hell is not the worst sin; and one the worst sins is the one least commonly committed
Atheism is infinitely more evil than immodesty, just saying!