Well there's dualism - where you believe everything is opposite and exclusive - black and white, up and down, good and bad.
Then there's non-dualism which basically says all of the separations between everything that make them dualistic are not really there and don't exist. It suggests we are still in the world and yet the world is a single unified whole. Everything here is connected. This alone can be a huge revelation.
But the course in PURE non-dualism, which actually means, if you take it to the logical extreme, if everything is REALLY totally unified then even the separations between objects, forms, space, time, all of those are also illusions... and therefore there really is only ONE thing that's real - God. In this view, there is no world at all - it doesn't exist. No real bodies, no real people, no real individuals. That's a really hard pill to swallow for anyone with an ego, and even for anyone who considers themselves `non-dualistic`.
Another word for non-duality is Oneness. But I think this can only be truly fitting when you equate PURE non-dualism with Oneness. Oneness doesn't mean there's two parts which are somehow resonating or joined or connected or sharing something. There is ONE. In pure non-duality, the truth is not only true, but NOTHING ELSE is true. It's not just a statement about what is real, but a clarity about what it not - and in this, there is no world at all - there is only God's reality.
Oneness was not a concept I could understand at all when I started with the course. All of my thinking was dualistic. My mind functioned in a dualistic way and I segmented the world dualistically. Even though I perhaps was on my way toward non-dualism, I still believed strongly that everything was compartmentalized and separate. It takes quite a long time and a lot of forgiving and healing and correcting of mind and a shift of your thought system over to Holy Spirit's thought system, for the mind to ... experience ... a sense of the connectivity of Oneness.
Now that I've absorbed some degree of a Oneness perspective, as I read ACIM stuff and reflect on its words, it does make a lot more sense now... because I can see and grasp more of how it is actually written FROM a perspective of Oneness. It's not just talking about non-duality, it was authored IN non-duality. Its viewpoint looks out with non-dualistic attitude and vision. Jesus teaches from a viewpoint of everything being One.
So as you approach the truth more and remove blocks to love, it's sort of like you rise up above the clouds and can start to SEE, and there is this openness and clarity as if looking down on things - "above the battlefield" as the course says. And up there, beyond the clouds of hate, there is light and connectedness and love. Love is oneness. It is everywhere.