The fool[a] says in his heart,
“There is no God.”
They are corrupt, their deeds are vile;
there is no one who does good.
— Psalms 14:1
Notice that the NIV Bible has a footnote “a” on the word “fool” and that footnotes is as follows:
Psalm 14:1 The Hebrew words rendered fool in Psalms denote one who is morally deficient.
"The fool has said in his heart “There is no God.” is NOT about atheists. So many of the claims which far too many Christians make about “atheists” based upon various scriptures betray a sad misunderstanding of the Bible. Psalm 14:1 is probably the most abused.
Bible readers should keep in mind that atheism in the modern day sense was not an issue in ancient Israel, to say the least, so nobody had much reason to talk about it. Secondly, if they read the Hebrew text behind the passage, they would notice that “the fool” is NABAL----which is also a name of a famous Old Testament fool who had a bad run-in with David. (See 1 Samuel 25.) Of course, the word had its meaning long before Nabal was born. (And by the way, it is not the same word at all as the Racca “Thou fool!” that Jesus prohibited.)
Long story short: The audience at the time knew what the text meant in saying “the fool says in his heart”. It is speaking of the religious people of ancient Israel who claim to know God but then —in their heart where they make decisions about right and wrong—they decide and act as if there is no God watching them. They may say with their mouth that they know God but in their moral/ethical decisions (say in their hearts) they act like someone who doesn’t think a righteous God is watching what they do, much less judge them for their evil acts.
Notice how this interpretation fits the context? Notice how it fits the story of the famous fool, NABAL? (1Sam 25)
If the ancient Hebrews had wished to speak of an atheist in our modern day sense, the passage would have referred to one who “has said with his mouth, ‘There is no God.’” Big difference.
SUMMARY: In that culture of ancient Israel …
“To say with one’s mouth” was to speak one’s thoughts and beliefs aloud.
“To say in one’s heart” was to reason and make decisions on a course of action (and actions speak louder than words.)
So, is the Psalmist talking about the atheists of our day? No. A modern day atheist in that sort of context in Psalms 14:1 makes zero sense.
The Psalmist saw that lots of people who outwardly spoke of submitting to the God of Israel as covenant members of the nation of Israel (under the Sinaitic Covenant between God and his people) nevertheless lived morally deficient, foolish lives because their inward thoughts and decisions were just as if they didn’t acknowledge God at all. That is why they they are called fools. Such a person is a NABAL.
The Psalmist had no reason to speak of a “says with one’s mouth” atheist. He had plenty of reasons to denounce “says in one’s heart” atheists among his own covenant people of Israel.
@Patrick, I hope that clears up that Hebrew exegetical issue that is often lost in translation.