I believe in..well, for a lack of a better term - Nature
It's more of an agnostic belief, I suppose, with Mother Nature being the personification of that belief.
Essentially, I experience the universe is 'alive'. It has a similar feel and energy, just infinitely greater, to any living being I've come across. For now, we don't really deal much with the universe in and of itself, and we spend most of our lives on Earth. To me, Earth is a living organism, just like the universe, just like you and me. And she is a mother to us all. We're like fleas on her back, I suppose. She is our life force, our home, our beginning and our end. Some would call that a Goddess, I guess. That to me, is their perspective and not a wrong one considering our position wrt to hers.
It's funny that Babylon 5 captured and articulated some of the beliefs that resonated with me. Delenn talks more than once about us being 'The universe made manifest'. I could see that being true - that we *are* the universe, a living being, that has split itself into multiple pieces in an attempt to learn all there is about itself. As she so poetically put it: 'We are star stuff.' We all possess that same 'living energy' that makes us connected, and powers this universe. That is at least how I experience spirituality - through feeling that connection, that energy not just in me, but in everything living around me. Interestingly, it is less or even not at all present in man made-objects ime, than things that are created in nature.
Considering energy never gets destroyed, just transformed (according to the laws of physics), one could argue that we are in fact, immortal. And that is an intriguing thought - to have mortality be a transformation to another state has been the basis of many a religion. There is something soothing and meaningful about that, really.