Psalm 53

Read Psalm 53 ESV

Who - David writes a poem for the people to hear.

What and Why - David does not appear to be speaking to God in this psalm but to the people of Israel, and the righteous worshipers of the LORD specifically.  (Though he does make a request of God at the end.)  He seems to want to instruct his people that the evil people around them are not to be feared. Yahweh will take action against those who reject God and pursue evil.  This psalm is very similar in content and message as Psalm 14.  

When - No specific time is given, but it appears to be when a foreign nation that rejects God has come against Israel.

Where - Unknown

How - David opens with pointing out the heart problem the fool has.  The result of saying God does not exist is evil and abominable actions.  David then reminds his hearers the LORD sees and knows what the wicked do.  However, David admonishes that no one is good and seeks God on their own.  Those who do evil do not have knowledge, ravage others and do not call upon the LORD.  Then David reminds his hearers that the LORD will take action against the evil person.  David ends with a call for the LORD to take action against those who have come against Israel.

So What 

1. Anyone who denies that God exists will do evil and abominable things.  Be on guard when around them or they are near you.  Additionally those who deny the existence of the God of the Bible will also do evil, just a lesser form.  
2. No one seeks the LORD on their own. While the Spirit works first, we must call them to seek Christ and then show them the way to the cross.  
3. While it appears the LORD is not stopping evil people who deny him, he will act.  At some point he will scatter their bones and put them to shame (Hitler).  Sometimes this will occur in this life as well as in the next, but sometimes it only happens in the next.  Even with those people we must remember 70 years here is nothing compared to eternity.