Almost every religion and spirituality, with some exceptions (e.g. Therevada Buddhism) requires a belief in spirit or spirits. A belief in spirit or spirits is in turn a belief in the existence of usually invisible minds that exist in a state that is separate or separable from brains and, usually, physical bodies. The individual spirit-minds may be named Gods, Angels, Demons, Devas, or Faery Folk or be embodied in natural objects e.g. trees or parts of the landscape.
It is easy to imagine how a belief in spirit and spirits came druing the development of human culture down the ages. Mankind, before the advent of modern science and technology did not understand the world and the forces of nature. Mankind now largely understands the unseen forces of energy, electro-magnetism, gravity, and biology that cause nature to work the way it does. Previously what science could not explain and did not have obvious physical form was understood as the result not of impersonal forces but willed events. Mankind's model was the human being who could effect the world, to some degree, at will. Some effects in nature were clearly beyond man's power e.g. a storm or plague to willl. But not beyond the power of beings greater than man, beings that came to be understood as 'gods'.
Yet a belief in spirits persists among religious believers, and those with animistic, ecclectic or esoteric spiritualities outside the main world religions. A belief in spirits is certainly fundamental to any belief in an afterlife that is part of most religious believes. Usually survival post mortem depends on a notion that each person has a 'spirit' that is not identified with their body, but instead usually identified with the person's mind. This spirit can exist in a disembodied state or at least can exist after the destruction of earthly body and brain. Allied to a belief that a spirit can be disembodied is the animistic notion that spirit or spirits can reside in all manner of creatures, objects, even parts of the landscape providing a form of consciousness without the structures normally associated with a brain.
But if spirit exists independently of physical bodies and brains, it is never explained what spirit 'is' . There are forms of matter and energy that are very strange, that are weakly interacting with the physical world and invisible. They have been shown to exist through research in physics and cosmology. However science has never demonstrated that such invisible (in the everyday sense) forms of matter or energy can create or constitute a mind. While believers would insist that as science is limited to detecting and measuring the physical world it is not surprising that the intruments of science cannot pick up on the existence of 'spirit'.
It is not of course beyond the bounds of possibility that minds could exist in forms other than exist in living creatures, in particular in humans. However most religions insist that the human mind is an example of spirit operating, but there is much scientific evidence that mind is dependent on a physical brain. Most modern scientists think the notion of spirit is a redundant hypothesis in that all natural and mental processes can be adequately explained and indeed much better explained in terms of their interaction with the environment without reference to spirit entities.
We must then explain why a belief in spirits persists when they offer no explanatory power and there is no accumulated, reliable physical evidence. The most likely reason is that we need a belief in spirit to indulge our desire for an afterlife. Admitting that spirit is impossible is admitting that we are limited to a physical and finite world.