Here's an article that may interest your, and spur you to investigate further....
Learning From Sartre - John T. Mullen
It's too long to paste in it's entirety, but this section might grab you...
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Anyone who has ever seriously committed himself to following Christ and conforming to His character quickly discovers how difficult it is to do. There are hindrances everywhere, but the greatest of these is the sin within the disciple himself. Indeed, the motivation for following Christ in the first place is to be rid, eventually, of the sin that destroys life and offends God. Hence we are exhorted to turn from our sin, which we do by ceasing from various activities that we know to be sinful and by undertaking others that we know to be good. So far so good, but there remains a nagging uneasiness. Our behavior may be better, but how much real growth in holiness has taken place? The feeling that we have only scratched the surface of this problem creates a deep desire to get to the bottom of our sin, to start attacking it at its very core. But how? What exactly is the very core of sin? If we knew this, we would certainly be better equipped for the attack.
Christian theologians have often addressed this question. The most notable example is Augustine's description of his stealing pears in his youth, a passage that has long been widely read in the Western world. Augustine was struck that it was the very forbiddenness of the act that caused him to take such delight in it; the pears themselves were no attraction at all. His analysis is a chilling anticipation of Sartre:
So all men who put themselves far from [God] and set themselves up against [Him], are in fact attempting awkwardly to be like [Him]. And even in this imitating of [Him] they declare [Him] to be the creator of everything in existence and that consequently there can be no place in which one can in any way withdraw oneself from [Him]. . . . And was I thus, though a prisoner, making a show of a kind of truncated liberty, doing unpunished what I was not allowed to do and so producing a darkened image of omnipotence?
Augustine realized that the essence of sin is to place oneself in God's rightful place, to attempt to be like Him in ways impossible for one of His creatures. Usually, such attempts involve a denial of God's authority to command His creatures and to set limits on their behavior. Sometimes, all creaturely limitations are thrown off. Sartre, as we shall see, took the latter approach.
If the true nature of sin has been identified for so long, it might be asked, what can an atheist like Sartre possibly contribute to our understanding of it? His "contribution" consists in turning the very essence of sin into the foundation of a philosophical system. He concedes as much when he tells us that "Existentialism is nothing else than an attempt to draw the full conclusions from a consistently atheistic position." Or again, "Man is nothing else but what he makes of himself. That is the first principle of existentialism." As he develops his thought, we begin to see how sin has infected us in ways we are not even conscious of. This is handy information for anyone whose highest desire is to turn away from sin, and it keeps one focused on what sin really is. Sartre is, of course, perfectly oblivious to this assistance he is providing for the Christian church.
The cornerstone of his philosophy is the sovereignty of human freedom. He is quite frank about what he means by freedom. For Sartre, freedom is nothing less than the power to define one's own being, to determine what one is. Anything outside oneself that exerts any influence over one's being is by definition an obstacle to freedom. He explains: "It is therefore senseless to think of complaining since nothing foreign has decided what we feel, what we live, or what we are. Furthermore this absolute responsibility is not resignation: it is simply the logical requirement of the consequences of our freedom."
This leads Sartre to distinguish between being-in-itself, which lacks freedom and cannot choose what it will be, and being-for-itself, which is continuously determining itself and hence has no fixed essence of its own. Man, says Sartre, is the latter: "There is no human nature, since there is no God to have a conception of it." This means that Man is in a constant process of becoming what he now is not. Since Sartre cannot say that Man ever is anything at any particular time, he equates Man's being-for-itself with nothingness. It is amusing to note that those who begin by assuming the sovereignty of human freedom must go on to conclude that they are as nothing. But it is more important to note that Sartre's assumption is arbitrary. It is the starting point for his speculations, for which no defense is ever given.
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The author is clearly a Christian, and rejects Satre's arguments; however, the fact that he still finds them to be useful might impel you to want to investigate then further....