I would be careful about saying that there is no God. Some existentialists believe that God created us, just without purpose. Kind of like a really cool science experiment. lol.
In waiting for Godot you're looking at Sartrean existentialism, based on his famous dictum 'existence precedes essence' i.e that we were created without meaning or any fundamental core being (i.e essence) and so, by our existence, and our actions, we define our essence for ourselves.
Linking this to Godot, you would say that the minimalist set implies that the actors have to create the meaning, whilst each character is at a loss to define themselves in the meaningless void they live their lives in. Didi, who comes off as more intellectual, has a much harder time with this knowledge than Gogo, who really doesnt care. That shouldn't be hard to find quotes on.
The thing you want to read is Sartre's lecture '
Existentialism is a Humanism'. It really lays out the facts albeit in a very verbose manner which takes time to get used to.
Hope that helps.