Quoting from Sartre in Simone de Beauvoir's 'Adieux: A Farewell to Sartre' (Translated by Patrick O'Brian; Penguin; London:1984) p 436 - 443 ... interviews with Sartre just before his death
[DB = De Beauvoir S = Sartre]
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DB: Among these friends were there any who tried to persuade you - I don't say convert you - but to persuade you of God's existence?
S: No, never. .....
DB: There was a time when you knew some Christians very intimately, and that was in then prison camp. Indeed your best friend was a priest.
S: Yes, most of the people I mixed with there were priests. But at that time, in the prison camp, they represented the only intellectuals I knew. ....They were intellectuals, people who thought about the same things as I did. Not always as I thought, but even so reflecting upon the same things was a bond. ... The Abbe Leroy told me quite spontaneously that he would not accept a place in Heaven if I were turned away. ...
DB: And when you wrote Being and Nothingness did you vindicate or try to vindicate your disbelief in God philosophically?
S: Yes, of course, it had to be vindicated. I tried to show that God would have to be the "in-itself for itself," that is, an infinite in-itself inhabited by an infinite for-itself, and that this notion of "in-itself for-itself" was it self contradictory and could not constitute a proof of God's existence. ... In Being and Nothingness I set out reasons for my denial of God's existence that were not actually the real reasons. The real reasons were much more direct and childish - since I was only twelve - than theses on the impossibility of this reason or that for God's existence. ... Even if one does not believe in God, there are elements of the idea of God that remain in us and that cause us to see the world with some divine aspects. ... I don't see myself as so much dust that has appeared in the world, but as being that was expected, prefigured, called forth. in short, as a being that could, it seems, come only from a creator; and this idea of a creating hand that created me refers me back to God. ...
DB: Apart from the feeling of not being here by chance, are here other fields in which there are traces of God? In the moral field, for example?
S: Yes. In the moral field I've retained one single thing to do with the existence of God, and this is Good and Evil as absolutes. ...
DB: Or as Dostoievsky says, "If God does not exist, everything is allowed." You don't think that, do you?
S: In one way I clearly see what he means, and abstractly it's true; but in another I clearly see that killing a man is wrong. .... I look upon the absolute as a product of the relative, then opposite of then usual view. ... it is certain that the notions of absolute Good and Evil arose from the catechism I was taught. ... That's what I mean. I think the objects I see here do indeed exist apart from me. It's not my consciousness that makes them exist. They don't exist for the sake of my consciousness and merely for that; they don't exist for the sake of the consciousness of mankind and merely for that. They exist without consciousness in the first place. ....
DB: When a man like Merleau-Ponty ... said he believed in God, or when your friends the priests, the Jesuits, said that they believed in God? On the whole what do you think the fact of stating that he believes in God represents the way a man leads his life?
S: ... At present .. there is no intuition of the divine. I think that nowadays the notion of God is already dated. .... They have a vision of the world that belongs to a past age. ...
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Sartre's above proof of God's non-existence also requires a thorough knowledge of philosophical terms that most Christians have no idea about. Sartre wrote in French and so as English speakers and reader we only have a very poor translation to work with. Namely:
"Being is. Being is in-itself. Being is what is is." Being includes both Being-in-itself and Being-for-itself, but the latter is a nihilation of the former. As contrasted with Existence, Being is all-embracing and objective rather than individual and subjective.
"Being-for-itself" is the nihilation of Being-in-itself; consciousness conceived as a lack of Being, a desire for Being, a relation to Being. By bringing Nothingness into the world the For-itself can stand out from Being and judge other beings by knowing what it is not. Each For-itself is the nihilation of a particular being. It is the human way of Being which is fluid and open to possibilities and imagination.
"Being-in-itself" is non-conscious Being. It is the Being of the phenomenon and overflows the knowledge which we have of it. It is a plenitude, a fixed and complete being, and strictly speaking we can say of it only that it is. It has no relation to itself or to anything else.