“By way of contrast, Sartre expresses the basic tenet of existentialism in this way:Existence precedes essence.In this statement he is taking existentia and essentia according to their metaphysical meaning, which from Plato’s time on has said that essentia precedes existentia.Sartre reverses this statement.But the reversal of a metaphysical statement remains a metaphysical statement.With it he stays with metaphysics in oblivion of the truth of being.For even if philosophy wishes to determine the relation of essentia and existentia in the sense it had in medieval controversies, in Leibniz’s sense, or in some other way, it still remains to ask first of all from what destiny of being this differentiation in being as esse essentiae and esse existentiae come to appear to thinking. [....] Sartre’s key proposition about the priority of existentia over essentia does, however, justify using the name ‘existentialism‘ as an appropriate title for a philosophy of this sort.But the basic tenet of ‘existentialism‘ has nothing at all in common with the statement from Being and Time — apart from the fact that in Being and Time no statement about the relation of essentia and existentia can yet be expressed, since there it is still a question of preparing something precursory.” (p250r, link below)
Existentialism is a Humanism is making its rounds on goodreads.As it deserves.I got my toe wet first in the Sartrean dark waters.But it is written from the ground of an (inevitable) misunderstanding of Heidegger's fundamental concepts.Instead of reading all of Being and Time in order to gain that correction, one can read his Letter on Humanism which functions as a kind of response to Sartrean existentialism.(And if you have it on hand, Derrida's "The Ends of Man" in Margins of Philosophy).
The Letter is available on-line ; merely 38 or 60 pages ::
Frank A. Capuzzi's translation from Pathmarks (photostat) :: http://pacificinstitute.org/pdf/Lette...
Miles Groth (unauthorized translation) :: http://wagner.edu/psychology/files/20...
Also, but if you really don't like Heidegger and refuse to ever read him for whatever reason ;; please, as you read Sartre, also look back to the Existenzphilosohie of Karl Jaspers.