Existentialism Is a Humanism is a work based on a lecture given by the author and it reads as such. It begins with the author declaring his purpose in the lecture, which is defending the existentialism against some charges made. These charges, as we discover, are mainly from religious people, Christians, who ridicule and sensationalize the idea associating it with something dark and ugly.
The author, then, starts to define existentialism and defend it against the accusations made. It is defined through the idea of abandonment, meaning that humans are responsible for their own existence and their own choices. It follows the atheistic path, even though existentialism isn't so much focused on the denial of God.
The author also makes a point about there being essence before existence. This means that there has to be an idea of something before something is brought into existence. In the case of humans it is the opposite: the existence precedes the essence. Humans are responsible for creating and shaping their own essence.
The author further discusses the human condition defined by existentialism talking about freedom, choices made, and gives examples out of real life, as well as make allusions to other authors and their works.
The work is finished with drawing a connection to the humanism, but a different kind of humanism where man is not the end goal but always a project in the making, and that man should project himself outwardly rather than inwardly to be truly human.