Ps.19
The heavens declare the glory of God; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language, where their voice is not heard. Their line is gone out through all the earth, and their words to the end of the world.
Acts 14:
But when the apostles Barnabas and Paul heard this, they tore their clothes and ran in among the multitude, crying out and saying, “Men, why are you doing these things? We also are men with the same nature as you, and preach to you that you should turn from these useless things to the living God, who made the heaven, the earth, the sea, and all things that are in them, who in bygone generations allowed all nations to walk in their own ways. Nevertheless He did not leave Himself without witness, in that He did good, gave us rain from heaven and fruitful seasons, filling our hearts with food and gladness.
Rom.1
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
It seems clear that all men do know God at some level, so does that exclude there being an "honest" atheist? Can such a person exist?