Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
Professing themselves to be wise, they became fools.
If they didn't not know it, then why the active hostility for the idea of a God? The very fact that these foolish people have to define themselves by the knowledge that they suppress proves that there is something that they are running away from....namely God. How can you not believe in something if that something doesn't exist? All ideas or images imagined come from something that exists. Even fantasy creatures, aliens, and the pagan gods are images formed by men who try to ascribe some form of something created to describe it. Everything we see has an origin therefore everything at it's origin must have had an Originator. How could the idea of a Sovereign God have been invented by those who would hate that very thing they have invented?
The thought of a God who holds them accountable for their doings, Who requires something of them did not come from self-serving, self-loving, self-glorifying fools. Man would never invent such a source of torment on his own conscience. So, for someone to define himself as an "atheist" is someone who just proves the point that he hates the fact that a God is ever present, ever watching and ever remembering his wickedness. By pretending that he doesn't exist, is as stupid as putting on blindfolds and saying that he doesn't believe in the sun.
Christian, don't be afraid of those who call themselves "atheists" the burden of proof that they are right is on them, not you. All effects have an originating cause, without an intelligent, all powerful God who has the power of self existence and self-sufficiency, how could all things have their existence? Eternal cosmic dust? or eternal God?
Eternal dust you say that created a big bang? Then you prove your foolishness in that answer.