It doesn’t make sense to use the words “I can’t, or I must” to something, because we ultimately do have a choice for everything we do.
When we act in a certain way, we can not blame it on anyone but ourselves, and we must take full responsibility for our actions, even though we may not really want to.
Even if Sartre believed in a God, it still wouldn’t help what he is trying to express.
If God existed then some people would just want to let God make their decisions for them, which gives people an excuse for their actions, and takes away all sense of freedom.
Or people would have to decide if they wanted to follow God’s rules in the first place, and would then have to be responsible for this decision.
However, there are consequences for this side of the argument as well.
Sartre’s main point is that from the moment we are thrown into the world, we must be completely responsible for all of our actions.
There are no eternal values or ethics that we can base the way we live off of.
This is how we need to understand man – that our “essence” is not something that is fixed in advance, but rather that each man creates this himself.
Humans exist in a different way than any other thing.