In spirituality, what we call "nonduality", you are basically seeking wholeness, the lack of separation. So either wholeness is presently missing, or you just don't know wholeness is already the case.
In the first scenario, wholeness is missing. If it is missing, what can we do to bring it about? What action can bring about wholeness? What force can superglue the universe of separate parts together? Is that what you mean by wholeness? A comglomeration of all the parts? Not really. You really are searching for the oneness of reality, yes? So if it's presently divided, spiritual seeking isn't really going to get us anywhere.
The other scenario, really the only other possibility, is that wholeness is already the case only you don't know it. In this case you take yourself to be an individual part, separate from all other parts, but that really isn't true. In this case spirituality isn't trying to affect any change on reality but it is a search for truth, for knowledge, for certainty about reality. It is trying to come to the truth about reality. In this case, reality is whole. Therefore spirituality is merely trying to eradicate ignorance about the nature of reality, about the nature of what-you-are, and come to realize that it is already whole and you are that wholeness.
So in the first case reality is split and there really isn't anything we can do about it. Spirituality is useless as is any other activity in bringing what is inherently divided into oneness. In this case the search is over because it is found to be a fantasy.
Yet we undertake the search as if we ARE separate and then trying to make it whole. Do you see the nonsense of that? The only activity which has any possibility of success is to inquire into the nature of reality, into the nature of your own self. It isn't a practice to try to change anything - we need not manipulate thoughts or feelings. It is diving into our ideas and trying to find out how we are coming to take reality as divided, how we are coming to take ourselves as divided - NOT trying to make ourselves whole or make reality whole.
What we call "enlightenment" is simply coming to the certain knowledge that you ARE the whole, not finding a new experience or state but the elimination of ignorance about your true nature.
Shankara said - ultimately there is no difference between Brahman and Individual Self. We must inquire into what we believe about ourselves and reality and if we're honest and earnest, most of our concepts will fall away as false - the root concept is the idea of separation.