Those verses came to my mind too. Maybe the true definition of atheist is not one who does not believe but rather one who is in denial.
What Paul said is sort of complex, but he stated.
1) They clearly did see
2) God did reveal to them
3) All things made that are invisible, are Clearly understood by the things made.
4) They have no excuse, did not retain God in Knowledge, despite creation itself screaming "There is A God!!!!"
Paul said their imagination darkened, and they were not thankful.
Not being thankful, carries a whole other list of heart issues. Paul continued on though.
Professing themselves to be wise, they became fools, And changed the glory of the uncorruptible
God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
(Rom 1:22-25)
5) They thought they were wise.
6) They changed the Glory of God which they saw through creation, and brought it down to the corruptible understanding of man.
7) Since the Truth of God did not set well with what they wanted to do, then they are not responsible if all creation is just as a image of man, whom they have no responsibility to.
Serving the things created and not the creator. (Blake mentioned this)
A whole lot of self deception and complex thinking a person has to go through to claim they are an Atheist.