The psalm begins with a blunt statement: the fool says in his heart that there is no God. They are corrupt and have done abominable deeds; there is none who does good. The word fool in the Bible describes a person who refuses to live under God's authority, and the unbelief it describes is practical.
Then the Lord looks down from heaven on the children of man to see if there are any who understand and seek after God. His verdict is that all have turned aside and become corrupt together; there is none who does good, not even one.
David asks whether the evildoers have no knowledge, those who eat up my people as they eat bread and do not call upon the Lord. There they are in great terror, for God is with the generation of the righteous. They would shame the plans of the poor, but the Lord is his refuge. The psalm ends with a longing: oh, that salvation for Israel would come out of Zion! When the Lord restores his people, Jacob will rejoice and Israel will be glad.