Spiritual USED to refer to a relatively coherent and rational schema of philosophical thought. That was out when the Transcendentalists - a philosophy much-loved by the Hippies of the 1960s - began their insinuation into organized religions. From Wikipedia:
A core belief of transcendentalism is in the inherent goodness of people and nature. Adherents believe that society and its institutions have corrupted the purity of the individual, and they have faith that people are at their best when truly "self-reliant" and independent.
Transcendentalism emphasizes subjective intuition over objective empiricism. Adherents believe that individuals are capable of generating completely original insights with as little attention and deference to past masters as possible.Originally brought into America by the Unitarians, this philosophy has infiltrated many seminaries and theological college. Major figures in American Transcendentalism include the Thoreau, the Alcott family (Louisa wrote the Little Women series), and Emerson. The origins in America borrowed from European thinking (Kant and Hume, among others).
So, what is going on with those people who call themselves 'Spiritual'?
For some, their experiences with organized religion have soured them on the institutional part of it, however, they still want a connection to God. This leads them to quasi-religious practices, such as meditation (non-Hindu), use of music and/or drugs to transcend reality, or other means to create/maintain a spiritual connection without having to conform to a doctrine or the practices associated with it.
It is a reliance on the internal feelings/thoughts as evidence of a 'God Connection', but not joining with others in a communal activity. It is religion as individualism, rather than as it has traditionally been practiced - as a group.
Perhaps ironically, some of those Spiritual Individuals pursue community connections through involvement in:
- Black Lives Matters
- Anti-Fa activities
- Environmental activism
among other things. They enact a rigid philosophy that punishes deviance from accepted wisdom, 'shun' non-conformists, and otherwise act similarly to the most authoritarian religions.
The Spiritual-But-Not-Religious folks often end up hectoring members of an organized religion about their failings, not to improve them, but to reinforce their own sense of self-righteousness.