Søren Kierkegaard is generally considered to be the "father of existentialism". This always bothered me, since to me Sartre and Camus are the defining figures of the movement, and it seems that there is no room for God in an existential worldview.
Kierkegaard was focused on concrete reality, and the individual, so you could say that there are elements of existentialism in his philosophy; but is that enough, given that he held a faith in a power outside the self which provides immortal value?
Perhaps a better question to ask is the inverse: whether traditional religion is compatible with existential philosophy; Kierkegaard's existential beliefs certainly affected his view of the role of religion, to the point that his theology was extremely controversial among his contemporaries. Existentialism doesn't necessarily require the outright rejection of the possibility that a God exists; just the rejection of the notion that we must place God before all else in choosing how to live our lives.
God is negotiable under existential tenets, as is any other exigency. Just as neither the weather in Brazil nor the affairs of Peter Pan bear much impact on a day to day accounting of life for me, neither does god; it is not to say the same for others' accounting.
An existentialist philosophy is nothing more than a philosophy who's subject is human life and the human experience. The substance and particulars of existentialism cover a wide spectrum of beliefs and ideas.
Much of the answer to this depends on what you mean by 'atheism.' As the opposite of 'theism,' that form of atheism is just as dogmatic as what it seems to reject. The existential project looks at the event in the present in all its contingencies, so our relationship to a deeper or 'divine' reality might well be part of that. This a/theism does not affirm that there is a transcendent being who/that created the world and now controls what happens on it--that is a difficult reality to justify in the face of ever-increasing evidence of the uneveness or unjust realities that just happen...such a vision of the divine has to affirm the incompetence of the divine agent. But that does not mean that there is not a deeper or other dimension to reality which is related intrisically to our actions. Quantum physics, string theory, and the worke of Brian Greene and others have opened up this possibility, not as a necessity, as Hawking pointed out, but as an enrichment of the existential moment.
For Kierkegaard, and by extension Christian existentialists in general, it is precisely the primacy of the personal relationship with God that releases the individual from all other bindings of religion, law, custom, morals and tradition (while at the same time laying on the existential "yoke" of absolute direct obedience to God).
Although this clearly places the Christian existentialist at odds with the religious bureaucracies of the world, it is not as much of a break with the Christian legacy as might be imagined --much of the writings of Paul and the teachings of Jesus in the gospels seemingly imply or arguably even demand as much.
Is this truly existentialism as commonly understood? Perhaps not, but the influence of Kierkegaard on Sartre and Camus is impossible to ignore. Both Kierkegaard and the French philosophers focused on the responsibilities of absolute freedom. In Kierkegaard that freedom is found in God, for Sartre and Camus, in the absence of God.
The religious existentialists like already mentioned Buber, would be studying the nature of religion in a different way. No existentialist would claim that these experiences are limited to the clergy, yet there is a certain supra philosophical import to say Kierkegaard, as if he were not just doing a philosophy of religion, but asking the reader to become something or other.
Which is why characterizing Heidegger as an existentialist may be problematic.
So in effect K. is both doing the philosophy of religion, and proselyting for it. Unlike e.g. historical Buddhists who aren't really philosophers in the "western" sense.
One thing that bugs me, though, is when people assume an existentialist is religious because they themselves are.
You cannot answer the question whether there is room for a "God" in the existentialist worldview or many other worldviews, if you do not first define what "God" is. Most people tend to skip that step, yet this step is IMO the most essential step in any argument. Once you restrict your definition of "God" to a definition that is fully compatible with Atheism, the issue becomes trivial and the answer is an obvious "yes".
And yes, you can actually define "God" in a way that is fully compatible with Atheism. Atheism and Animistic Pantheism as equally valid descriptions of reality, different from one another only at the superficial level of semantics. As such, Atheism and Animistic Pantheism can be used interchangeably and should be treated as the same.
Concepts of divinity that are not Animistic are fundamentally flawed and don't belong in any rational worldview, existentialist or not.
An Atheistic perspective on Consciousness
We know from evolutionary biology that multi-celled organisms evolved from single-celled organisms. It is unclear to what degree single-celled organisms gave up their autonomy to be able to act as a single organism and it is reasonable to suggest that our individual cells have maintained some degree of autonomy (consciousness) that we are totally unaware of.
A lot of human behavior is associated with subconscious processes in the prefrontal cortex that psycho-analysts refer to as the super-ego. This super-ego reflects the internalization of cultural rules in the form of memes. Such memes often influence human behavior in ways individuals barely realize and can be considered a form of collective consciousness.
A group of humans that is connected by means of memes can act as a single conscious organism, much like a cell of our body can act as a single conscious organism. This and many other factoids directly imply that consciousness is not so much a product of our brains but rather a product of complexity and connectivity.
From that perspective, the concept of consciousness can both be reduced to the molecular level and expanded to the universe as a whole, with the universe as a very complex holographic quantum computer
The relationship with Animism
In Hinduism, death is understood as the collapse of the Ātman (individual consciousness) and its dissolution into many different other components that make up the Brahman (universal consciousness).
If you look consider the Trimurti (the Hindu triniti), one can clearly see them as anthropomorphic representations of nature (Vishnu) and its two fundamental opposite forces: emergence (Brahma) and entropy (Vishnu). Similarly, many other Devas (Gods) are mere anthropomorphic representations of lesser natural phenomena.
The same applies to the Kami of Shinto religion or equivalents in other "polytheistic" religions. One could easily argue that all "polytheistic" religions are really Animistic religions, which are perfectly compatible with the Atheistic framework depending on how one defines concepts like "Consciousness" or "Soul".
Pantheism and Shamanism as forms of Animism
Pantheism is a simplified version of animism, which removes all "Gods" from the picture except Vishnu (nature). Advaita Vedanta is a form of Hinduism that belongs to this category. Many other religions (eg. Germanic paganism) have a Pantheistic variation, although these advanced forms of religion are rarely known beyond a small esoteric circle of initiates. The Traditionalist School is a school of philosophers from the early 20th century that attempted to explore these esoteric religions and discover a perennial philosophy running throughout all religions. Like Animism, Pantheism is also perfectly compatible with Atheism, again depending on how one defines concepts like "Consciousness" or "Soul".
Shamanism is a variation of animism that involves the notion that the chemical modification of one's consciousness provides access to higher knowledge that is otherwise filtered from our perception. Shamanic practices do not require a belief in any "Gods" and are not uncommon among Atheist intellectuals, especially since the popularization of LSD and Mescaline in the '60s. Aldous Huxley, Ernst Jünger, Alexander Shulgin, Timothy Leary and Terence McKenna are among the more prominent shamans in Western culture, although not all of them would use the term "shamanism" in reference to their chemically induced self-exploratory consciousness expanding sessions.
The Pantheistic "God" concept from the Atheist's perspective
The Atheistic naturalistic position of the universe as a very complex quantum computer is pretty much the same as the Pantheistic position of the universe as one highly advanced mind. The differences between Atheist naturalism and Pantheism are really more a matter of different semantics and different cultural bias than a difference in concept.
If one considers the notion that the clock ticks of a computer and awareness are basicly of the same nature (which is a perfectly reasonable consideration), the following statements would mean the exact same thing :
The universe is God. By being a part of God, all matter and living beings are essentially divine. Time is our perception of his thinking process. God's thinking processes comprise of all our thinking processes and all other processes of the universe combined. Our awareness is a tiny fraction of divine awareness.
The universe is a giant computer. By being operating systems somewhat autonomicly operating a part of that computer, all living beings are components of the same computer. Time is our perception of the giant computer sequentially processing information. The computer's sequential processing comprises of all our thinking processes and all other processes in the universe combined. Our awareness is but a tiny fraction of the universe's operating system.
So if time = awareness = clock ticks, I can't distinguish between those two statements. Conceptually they mean exactly the same.
The disconnect between Atheists and Theists largely stems from using very different semantical contexts to really describe the same perspective. If we were to adjust our semantics more to each other, many of us might see more similarities than they ever held possible.
Under the extension of existentialism that I've developed (e.g., here), atheism is not required for the broader existentialist philosophy. For me, existentialism is foremost the philosophy of existence. Existence might precede essence, and God(s) or Goddess(es) just might necessarily precede existence.
