I'm not convinced that the word 'spirituality' in its modern usage can be equated with belief in 'spirit beings'.
The 'spirit' family of ideas has a long and fascinating history. In ancient times, the word originally meant something like 'life force'. It was imagined as what animated living bodies, made them capable of moving on their own, and made them conscious and aware. Spirit was often identified with the breath, since death occurred when breathing stopped. We have the imagery in the Bible and elsewhere that had creator gods "breathing" life into inanimate bodies to give them life. And there was an idea that one's life force could drift off like a gas, either during 'out-of-body' experiences or when death occurred. So the idea of ghosts and vaporous spectral spirits appeared. Angels and even gods themselves were sometimes imagined as spirits of this sort. We see the idea developing in the Jewish tradition that God's breath or 'spirit' is kind of an intermediary between heaven and this world. That suggested Christianity's idea of the 'holy spirit', which naturally directed attention to the idea of people who had supposedly been touched by this holy spirit. And that idea of people infused with the holy spirit was associated with the early emergence of the idea of spirituality.
In the modern period, the early Christian idea of spirituality has changed dramatically. Emphasis moved away from pneumatology, from a concentration on the divine gas that had supposedly flowed out of heaven and entered into and animated living things, particularly the Christian reborn and reanimated by the holy spirit, to a new concern with the behavior that these Christians were supposed to display. The whole idea of spirituality became psychologized as the metaphysical theory of pneumatology faded away and was replaced by modern biology's understanding of life.
This new psychologized idea of spirituality has gradually become disassociated from the Christian tradition and has broadened out to the point where people today speak easily of all varieties of non-Christian spirituality. What the word 'spirituality' seems to suggest today is the idea of inner transformation. Some people see the meaning of 'spirituality' as being broader than 'religion', which can suggest the doctrines of a single narrow faith tradition. Others see spirituality as referring to the heart and essence of any and all religion, to religion's inner experiential core. Today the word 'spirituality' is even commonly used in more secular contexts such as psychology and art, where it typically refers to feelings of, and to the cultural artifacts capable of invoking, feelings of transcendence and/or psychological centeredness.
