This is by no means an exhaustive discussion of Sartre’s existentialism. I find Sartre’s views on human existence interesting, and in fact agree with him on some points. But there are some claims that seem problematic, and that is what I am going to explore.
According to Sartre, the human “existence precedes essence” (311). What this means simply is that human beings do not come to the world with a manual explaining why they were created or their purpose on earth, unlike a calculator would. Human beings simply find that they exist. But once humans start to act upon the world, they begin to create value, and in doing this give meaning to their lives.
The reason that the human existence, at first, has no meaning is because there is no God to define it. The theistic God, which is the God Sartre is referring to, is thought to be the creator of the universe. If such a being existed, then the existence of this being would precede the existence of man; and human beings would be his handiwork. If it were to be the case that humans were created by God, then human existence would be explained by divine purpose. Since Sartre believes this is not to the case, Sartre argues that the human existence at first has no meaning. Meaning is created by humans.
It is at this juncture that I disagree with Sartre. Sartre seems to be arguing that the fact that God does not exist makes the human existence devoid of meaning, until humans define themselves. Is the theistic God the only source of meaning that humans have besides themselves?
One does not have to subscribe to theism, in order, to believe that the human existence has an essence outside of it’s self. Neither does one have to seek to define his or her life, in order, for one’s life to have meaning. Human life has an essence, from a biological stance, whether or not humans know this or not. Humans are part of a biotic community, and human activities ensure the survival of that community. One can be an atheist, and conceive of human existence as having meaning in its role as a part of the biotic community. Also, note that such a definition of human kind is not created by man.
Sartre is giving humans only two ways to find meaning to their existence: God or themselves. I do not agree that humans have only two options before them. Or at least, let’s look for other options!
Works Cited
Sartre, J.-P, “The Humanism of Existentialism”, from Essays in Existentialism, ed. Wade Baskin, tr. B. Frechtman, Secaucus, NJ: The Citadel Press, 1972

I agree with what you are saying. What he is saying is faulty because that would mean that those who do not believe in God are devoid of meaning. Well, at least, I think that’s what you are saying.
I have not read Sartre in much detail. From what I have read and through the information you have presented, I can see how his philosophy has holes in it. To begin with, in order to say there are only two options to a problem requires proof that no such other solution exists. Furthermore, there is no way to know for sure either or.
Well, that’s what philosophy’s about anyway. Theorizing on things we can never know.