Statements like these are empty. Change "no" to "a" and the statement is equally as valid.
"The fool says in his heart there is a god".
Or turn the subject into something else.
"The fool says in his heart there are no unicorns".The validity of statements like these greatly depend on where those words come from and who is saying them. The line in the OP is from the Christian bible(s), and therefore, some people give it a lot of weight. I don't.
This little gem comes from Psalm 14, which in part goes:
The fool says in his heart,
"There is no God."
They are corrupt, their deeds are vile;
there is no one who does good.
The LORD looks down from heaven
on the sons of men
to see if there are any who understand,
any who seek God.
All have turned aside,
they have together become corrupt;
there is no one who does good,
not even one.
I don't believe non-believers are corrupt or vile. I don't believe that non-believers are inherently
not good. There are plenty of non-believers that do very good things. Invalidating the latter portions of this section surely puts the first in question.
So say it loud, and say it proud;
There is no god.