Sartre believed: If God exists man is not free, and if man is free God does not exist

Jean-Paul Sartre, (1905-80), French philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic, was one of the key figures in the philosophy of existentialism, and one of the leading figures in 20th-century French philosophy and Marxism. His work has also influenced sociology, critical theory, post-colonial theory, and literary studies, and continues to influence these disciplines.
The following short article, which appeared in a little known website claiming to focus on philosophy, explains Sartre’s worldview is explained, especially how it differs from his conservative contemporary, Martin Heidegger (1889-1976), also an existentialist:

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For Sartre human beings live in anguish, or the feeling of total and deep responsibility, not because life is terrible, but because, as he says: “Man is condemned to be free; because once thrown into the world, he is responsible for everything he does.”
We are born without a choice, yet here we are born into a world with so much freedom to choose while simultaneously held responsible for everything we choose to do in this existence that we didn’t choose to have. We are condemned to be free.
In Existentialism, this is known as thrownness, a word coined by Martin Heidegger. It is the condition of an individual’s existence upon being thrown into the absurdity of the material world, arbitrarily born into a given family, within a given culture, at a given moment in human history. Heidegger calls these “givens” facticities.
Sartre addresses that these limiting things that we don’t have control over do not limit our freedom. As he says: “freedom is what you do with what’s been done to you.”
Since existence precedes essence, there is no design for a human being, there is no God. For Sartre, phenomenology has to be atheistic. Assuming that God exists and has created everything would mean that essence precedes existence, the opposite of Sartre’s view. We therefore exist first and only then do we end up trying to make sense of things by way of science, religion, political ideology, philosophy, or anything else.
That is quite a difference from Kierkegaard’s view of Existentialism, for him, you can’t do Existentialism without God, for Sartre it works the other way around. As you can see, these existentialists have really different ideas even while pertaining to the same philosophical movement.
Sartre tries to rebuild the idea of freedom taken out of the Christian culture, getting rid of the power of God on human life. He believes that if God exists man is not free, and if man is free God does not exist. If God is dead, or as Dostoevsky said: “if God does not exist, everything is permitted.”
So, if there is nothing that preordains our human nature, then we must be free. We can then begin to set our own meaning to our life. Once we exist, it is our job to discover our essence. Freedom is one of the most important aspects of Sartre’s philosophy, to understand how truly free you actually are.
However, once you realise that you are completely free – you begin to feel dread of the amount of possibilities that are open to you, everything is possible.
It is you who has to decide the meaning of your life, when you realise that your freedom is completely without direction or guidance, it produces a sort of dizziness or nausea, which is why Sartre regards freedom as a condemnation.
Man is nothing but his life and actions, and this is horrifying.