What underlies unbelief? Some will tell you that they have examined the evidence, and found it wanting. Of course, that is a tenuous assertion, as the inductive methodology applied there can never lead to absolute truth. But even that doesn’t account for the underlying presuppositions that press one into scouting for evidence.
It is quite simple, and we find the explanation in the Psalms:
Psa 36:1 Transgression speaks to the wicked deep in his heart; there is no fear of God before his eyes.
Psa 36:2 For he flatters himself in his own eyes that his iniquity cannot be found out and hated.
Psa 36:3 The words of his mouth are trouble and deceit; he has ceased to act wisely and do good.
Psa 36:4 He plots trouble while on his bed; he sets himself in a way that is not good; he does not reject evil.
The unbeliever simply believes in himself more than anything or anyone else. There simply, in his presupposition, is no-one or nothing to be measured against other than himself. Wisdom is that which is in himself, and has no other source. He does not believe nor understand that there is any inherent evil or badness in himself.
A sad state of affairs for the unbeliever, and it can only be solved by asking for wisdom from the Creator.