There are those who are cocksure that there is no God, no soul, no post-mortem judgment, no ultimate meaning to human existence, and that we are all just material bits of a material world. Now it may be so for all we know. This is not an area in which proofs or disproofs are possible.
But for those who are cocksure about it, I suspect that it is their cocks that make them sure.
Crudity aside, their natural concupiscence blinds them to the spiritual reality of God and the soul, dulls their consciences, and ties them to a passing world that their lust convinces them is ultimately real.
This is why I do not trust the atheisms of Russell and Sartre. They were sensualists and worldlings who failed to satisfy the prerequisites of spiritual insight. Pride and lust dimmed their eyes.