What exactally is an Existentialist?
PaperYomiko is right, and although I haven't read any of Nietzsche or Sartre yet, I've been influenced by Albert Camus and Samuel Beckett, both who have slightly different views of existentialism.
In sum, this is how I interpret "agnostic" existentialism, and keep in mind there's multiple variations of this philosophy...
We humans, are on our own. Whether God exists or not, is not within our grasp to "know", therefore we should focus on the life on earth, and not think about an "afterlife" that may or may not exist.
Death is based on "chance". Anyone of us can die at any moment.
We can live good lives, or bad lives, but we will still die at any moment, based on "chance". Some good people die when they don't deserve it, and likewise, some bad people die when they do deserve it. In other words, death is "arbitrary".
On morality, an existentialist has a choice: you can do whatever you want, as long as you don't harm others (i.e. golden rule).
Otherwise, we all should be held accountable for any wrongdoings.
According to Albert Camus, the biggest "sin" to an existentialist is "not living". In simplest terms, to "live" is to live life according to how you envision it. We must create our own "life's meaning".
The biggest "sin", he says, is to "not live". For example, if you are unhappily working in your tiny cubicle of an office, then you're not living. Also, if you commit suicide, that is "not living" in the ultimate sense.
Likewise, you cannot kill others, because just as it is wrong to take your own life, it is wrong to take others. Some, but not all, like Albert Camus, is against the death penalty. Even when in prison, and you may think that your life is over, you must still continue to "live".
According to Samuel Beckett, life is "meaningless". There's no point, but we can't help the fact that we exist on earth, even if it's for a brief moment.
Also, we have each other. Make friends with other humans, it will help ease the anxiety of death. We will become less alienated, we will be able to "cope" with the "meaninglessness" or "absurdities" of life.
"Agnostic" is someone who is not entirely sure if God exists, and assumes that we lack information and knowledge about him.
Moreover, as an "agnostic existentialist", my interpretation and position is that when I die, physically, my body is still on earth, however, I will be unconsious forever.
Assuming God exists, I believe he created us, and then afterwards, is apathetic, or not "caring" about what happens to us, so that we are pretty much on our own.