The entirety of the spiritual search can be compressed to one sentence.
Are you what you have taken yourself to be?
This isn't an endeavor to achieve great spiritual states and experiences. This isn't a process by which your life is made better. This is not a self-help method. It is a single question which requires that you look directly into the root idea you have about yourself.
So we must ask - what is it that you presently take yourself to be? Try to get a real good grip on that. It's often very vague and foggy - we have never really stopped and asked this question - we have never really gone down that road in questioning - partly because it seems to be beyond question or doubt - and partly because it's a frightening and uncomfortable topic. We have spent a lot of energy sweeping that topic under the rug because we don't really want to think about it - it takes us down the road of pondering the end of our existence.
But that is exactly what we must look into. The beginning and end of our existence. So first get a good grip on what exactly you take yourself to be - you believe you are an entity which has a finite existence. You believe existence is partitioned - separated - owned. You have your own existence. Existence is something you can possess, in other words - Being is individual and finite.
So you, that entity, began on some day - at conception or during the 9 months of pregnancy or something. Some time during pregnancy you began - your particular existence began. It wasn't there before and then it was. By some magic or natural process you-the-entity sprang into existence from non-existence. You did NOT exist prior to that point.
Now you exist - you-the-entity exist - you stand alone in existence - the existence of a tree is not the same existence as your existence. You presently exist but at some point that will stop. That existence will end. You will cease to BE. You will no longer exist.
Isn't this exactly what we believe? Can you see that this idea requires independent existences - separate existences? This idea is at the very core or root of reality for you. Everything else is built upon that foundation. Your entire life story requires that idea to be true. Your entire concept of yourself requires that idea to be true.
At the very foundation of your idea of reality you take yourself to be a separate and finite entity. However you are now in a spiritual search for wholeness. You are looking for Oneness. You are looking to BECOME ONE. You are interested in something called Nonduality, when the very root idea you have about yourself is of duality - multiple existences which begin and end. Can't you see that as long as you hold this root idea beyond question, any spiritual search is futile and a fantasy? You wish to remain as this independent entity while GAINING some "state" called Nonduality or Enlightenment or Awakening or something.
When we get serious about spirituality, we may see this inherent contradiction and be compelled to actually dig into this idea - take it apart and see what it's made of. We might entertain the ideas we have about existence and see if it is something which begins and ends. Instead we may see that it is only FORMS or expressions or patterns which begin and end while existence itself never changes, never comes and goes. We may see that existence is what IS. And what you actually ARE cannot be other than THAT.
Before we can be free of a concept we must know what it is. Seriously consider what it is you take yourself to be. Get a good grip on that. Don't be afraid to question this idea.