Agnosticism, technically meaning 'the spirituality without knowledge', is a practice that puts all previous religions into question. Just as the post-modern movement desires re-evaluation of gender, sexuality, and concept, agnosticism calls into question the origins of god and the universe. While it would be completely arrogant for the human race to assume that they were the highest beings, power-wise, we have no proof that there is only one higher being. Further-more, we have no proof that they are out to do us any good, and no proof that they can or wish to communicate with us at the present moment. In essence, Agnosticism is the ultimate 'who knows, who cares', in terms of spirituality.
Now. Existentialism is, essentially, the contemplation of existence. Often the parable of Sisyphus is used as the existentialist mascot. The futile image that the parable creates is to symbolize our life...for, linking with post-modern strains of thought (which, remember, seek to re-evaluate all we have previously assumed to gain a higher truth), there is technically nothing more than this struggle. However, existentialism largely wished for us to accept this. Don't hate the struggle...it is ALL you have to depend upon. Your language, your concept of time, your concept of relationship, your concept of government, your concept of organization, your concepts of religion...all will fail you. However, the boulder will not. Accept and love the boulder.
Therefore, Existentialist Agnosticism combines these two views. We have no proof there is god. So, for your own sakes, stop worrying about him and live your life.
I am a self-proclaimed practitioner of the school of thought existentialist agnosticism preaches.
I feel that we all could learn from existentialist agnosticism.