It seems to me that existentialism is inherently connected with rebellion. Looking at some of its founders, we can note a certain pattern. There are revolutionaries like Kierkegaard and Nietzsche, who each in their own different way attempted to upturn the accepted moral standards and conventions and replace them with more immediate, personal beliefs and convictions.
The focus of existentialism is indeed individual existence itself. But it is one where you are, as Sartre would say, “condemned to be free” and “thrown into the world”, when you are wandering the long and wide road of life without a road-map or point of destination, except perhaps the inevitable event of one's upcoming death.
However, as you are in the midst of fear and angst you would like to hold onto anything that comes your way and gives you any kind of stability. People will offer you “road-maps for the soul” or they will try to fill you up with promises of heaven and the afterlife to give you some kind of security, some kind of grip on the absurdity and brutality that you are faced and grapple with on a daily basis.
Sure, faith does play a role in existentialism because reason seems incapable of fully answering the principal existential questions that are on the tip of your tongue. But it does not simply consist of embracing an appealing-looking faith; it involves struggle, suffering, endless-seeming battles and the ever-present and constant voice of doubt in your head. Faith is something one must earn over time and to have the scars to prove it.
But in the midst of this philosophical movement, there is an invitation for rebellion. Camus changed the famous slogan by Descartes from “I think, therefore I am” into “I rebel, therefore we exist.” Rebel against what? It goes back to the revolutionaries of old, a call to fight against all that is imposed upon us (against our will), to fight against ignorance, against bigotry, yet most importantly, against one's own deep-seated smug indifference.