From my humble point of view:
Dualism
Dualism is what we know from this world:
Hot - cold
Bright - dark
Good – bad
To be able to percieve anything, there need to be differences. There will be a subject (me) who percieves an object (everything else). So I can see you or the mountains in front of the window. If there are no differences, like everything is dark or bright, it’s not possible, to percieve anything.
On the level of religion, God is seen as good AND evil.
Non – dualism
This is, when we immagine/feel, that everything is connected. We are one with stones and stars, with humans and animals. Non–dualim can not be percieved in this world - due to the lack of differences between things.
There is a me that expands through everything else and so eventually everything ist the same as me.
On the level of religion, God doesn’t exist. (We are Godlike)
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In the book there are two more types of dualism:
Semi-dualism
Semidualism is between dualism and non-dualism. That’s when the idea grows that God is only love. However; evil can still be percieved. On this point arises the dilemma which can not be solved on this level; why is there evil, when God is only love?
On the level of religion you believe, that God loves you, but sometimes he still (seemingly) hates you.
Absolute non-dualism
That’s when you are aware, that there literally is nothing you could be one with except for God, because there is nothing but God. (Everything else is but a dream.)
On the level of religion, God is the only existing thing. We are a part of it and godlike.
If you like, you can try to feel what is living inside you. You might feel like everyting that bothers you in your everyday life goes away. You might feel great peace and a space inside you, that starts to dilate…
I was told, that this is what it is like to be «connected to God» I don’t know, wether it’s true, but i take this to work with. (Clearly God is also light ant love and so on...

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I hope, this was somewhat helpful..