What is spiritual realisation?

What is spiritual realisation? The belief that you are spirit? No, that’s a thought. A little closer to the truth than the thought that believes you are who your birth certificate says you are, but still a thought. Spiritual realisation is to see clearly that what I perceive, experience, think or feel is ultimately not who I am, that I cannot find myself in all those things that continuously pass away. The Buddha was probably the first human being to see this clearly, and so anata (no self) became one of the central points of his teaching. And when Jesus said, “Deny thyself,” what he meant was: Negate (and thus undo) the illusion of self. If the self - ego - were truly who I am, it would be absurd to “deny” it.

What remains is the light of consciousness in which perceptions, experiences, thoughts, and feelings come and go. That is Being, that is the deeper, true I. When I know myself as that, whatever happens in my life is no longer of absolute but only of relative importance. I honour it, bit it loses its absolute seriousness, its heaviness. The only thing that ultimately matters is this: Can I sense my essential Beingness, the I Am, in the background of my life at all times? To be more accurate, can I sense the I Am that I Am at this moment? Can I sense my essential identity as consciousness itself? Or am I losing myself in what happens, losing myself in the mind, in the world?

----- excerpt from A New Earth by Eckhart Tolle