The one thing that Sartre does get right in his 1946 lecture “Existentialism is a Humanism” (https://www.marxists.org/reference/archive/sartre/works/exist/sartre.htm) is this:
“Our point of departure is, indeed, the subjectivity of the individual, and that for strictly philosophic reasons. It is not because we are bourgeois, but because we seek to base our teaching upon the truth, and not upon a collection of fine theories, full of hope but lacking real foundations. And at the point of departure there cannot be any other truth than this, I think, therefore I am, which is the absolute truth of consciousness as it attains to itself. Every theory which begins with man, outside of this moment of self-attainment, is a theory which thereby suppresses the truth, for outside of the Cartesian cogito, all objects are no more than probable, and any doctrine of probabilities which is not attached to a truth will crumble into nothing. In order to define the probable one must possess the true. Before there can be any truth whatever, then, there must be an absolute truth, and there is such a truth which is simple, easily attained and within the reach of everybody; it consists in one’s immediate sense of one’s self.”
The other thing that he does get somewhat right in that lecture is this (which is the concluding part):
“You can see from these few reflections that nothing could be more unjust than the objections people raise against us. Existentialism is nothing else but an attempt to draw the full conclusions from a consistently atheistic position. Its intention is not in the least that of plunging men into despair. And if by despair one means as the Christians do – any attitude of unbelief, the despair of the existentialists is something different. Existentialism is not atheist in the sense that it would exhaust itself in demonstrations of the non-existence of God. It declares, rather, that even if God existed that would make no difference from its point of view. Not that we believe God does exist, but we think that the real problem is not that of His existence; what man needs is to find himself again and to understand that nothing can save him from himself, not even a valid proof of the existence of God. In this sense existentialism is optimistic. It is a doctrine of action, and it is only by self-deception, by confining their own despair with ours that Christians can describe us as without hope.”
Know Thyself 