Most people have an idea of what duality means. Usually a world of many separate things. Separate objects, separate bodies. A count beyond one.
This however isn't a full enough definition. Duality is also a matter of NOT SHARING. It's a state of mind in which you do not want to be joined with, share with, or have relationship with, anything or anyone. It's also a state of selfishness, therefore, because it opposes sharing. It produces isolation and disconnection. The ego thought system - the world of duality - is all about selfishness.
"Selfishness is of the ego" T-7.IX.1:4
If duality really means selfishness, what is the opposite of selfishness? It is sharing. But this requires that there be someone or something to share WITH. It cannot mean that there is only you there, or only God. It also requires that in absolute reality, in God, there must be something other than JUST God, such as your brothers, to share with. For example:
"You dwell in the Mind of God with your brother, for God Himself did not will to be alone." T-11.I.1:6
"Because He did not will to be alone, He created a Son like Himself." T-11.I.5:7
God wanted to share himself. In fact we're also told that SHARING is the natural inherent property of reality.
"Nothing real can be increased except by sharing. ³That is why God created you. ⁴Divine Abstraction takes joy in sharing. ⁵That is what creation means." T-4.VII.5:2-5
We're told in ACIM itself, putting aside all interpretations by other well-meaning teachers and students, in Jesus's words, that God did NOT will to be the "only thing that exists". He did not will to be alone. He willed that there be ANOTHER, and CREATED at the very least one son.
He even goes on to say that God not ONLY willed to create at least one son, this son ALSO has the ability to create its own sons.
"God would not have us be alone because _He_ does not will to be alone. ⁷That is why He created His Son, and gave him the power to create with Him. ⁸Our creations are as holy as we are, and we are the Sons of God Himself, as holy as He is." T-8.VI.8:6-8
Note also that it clearly states that God has SONS, not just one.
"The creations of every Son of God are yours, since every creation belongs to everyone, being created for the Sonship as a whole." T-7.IX.5:3
"The Kingdom of God includes all His Sons and their children, who are as like the Sons as they are like the Father. ¹¹Know, then, the Sons of God, and you will know all creation." T-7.XI.7:10-11
It's quite simply stating that God not only created many children, but that even those children in turn go on to create more children. The Sons of God form the sonship, which shares the One Mind of Christ. And collectively as the One Son, we all co-create with God to produce our own creations.
This is in fact why Jesus refers to us as having BROTHERS. The term brother means an equal sibling of the same parent. Because God did not will to be alone, and did not will us to be alone, he created MANY OF US, so that we would have permanent brothers IN heaven forever.
"Return with me to Heaven, walking together with your brother out of this world and through another, to the loveliness and joy the other holds within it." T-18.I.12:4)
"Here stands your brother with the key to Heaven in his hand, held out to you. T-24.II.7:6"
There is also a holy trinity in reality. God the father, the Sonship as the One Son comprising many sons and their creations, and the Holy Spirit. These are not conceptual or ego inventions, they are permanent.
"The Son of God is part of the Holy Trinity" T-3.II.5:4 "Your creations are your gift to the Holy Trinity, created in gratitude for your creation." T-8.VI.5:3)
The Holy Spirit is also a permanent eternal creation who will continue to exist along with the many sons even AFTER the atonement.
"When the Atonement is complete and the whole Sonship is healed there will be no Call to return. ⁶But what God creates is eternal. ⁷The Holy Spirit will remain with the Sons of God, to bless their creations and keep them in the light of joy." T-5.I.5:5-7
So what we have now is in fact a more fleshed out picture. Heaven, God's reality, comprises God, the Holy Spirit, the Sonshsip, which form the trinity. And the Sonship includes many sons, and many creations of many sons, who are also brothers. By the way there are angels as well, created by God.
Now, such a picture does not sound remotely like "non-duality." Someone came up with non-duality as an antithesis to duality, purely as a conceptual idea. It's an attempt to try to explain what the course is saying. Except that, it doesn't quite fit.
It fits in the sense that it may describe God himself, IF God were alone and willed to be alone. As a LOGICAL DEVICE, opposite to "many" means "one", a count of one. This is what non-duality logically means. There are many flaky and wishywashy definitions out there which have tried to glue toegether pieces of the course to make it fit with this, but the PUREST form of non-duality is that "there is literally only God."
This obviously does not FIT with what ACIM says, because it's told us that God is a creator, that he has created at least one child if not many, and that his child has has its own children. Not to mention also the holy spirit being created, and the trinity existing. All of this is not properly encompassed by the concept of non-duality.
Some people like non-duality ideas because they seem to suggest the antithesis to "separateness". As if it is the opposite of ego, the opposite of separation, and the way to undo all trace of the separation from God. But this is VERY BASIC logic and highly inaccurate in its representation of what JESUS ACTUALLY says.
The purest non-duality does not support God being a creator, or God sharing, or God having relationships, or God having even ONE son, or of God creating the Holy Spirit, or of YOU existing, or of you having any creations. It doesn't even allow for heaven to exist. All it does is reduce God to a singular blob, a very lonely isolated blob, which sits there all by itself doing nothing. That is NOT what God is like according to ACIM itself.
The problem is that many people have latched onto this non-duality "concept" and have tried to FORCE the course to fit into it, because they don't have a better OR more accurate model. And in many cases also because they don't recognize the shortcomings of this concept.
It's easy for anyone to say, well, if I get rid of all the separations which implied "many things", then I will get down to "one thing", and this is oneness, and it must be what reality is like. Except that, it isn't. Not by a long shot.
Not according to what Jesus has said about God ADDING to himself, and SHARING himself, and having REAL relationship with his Son, and HAVING a son, and CO-CREATING with him. Jesus's definition of ONENESS is not what you think it is. It is not the everyday definition. It's more to do with a SHARED WHOLENESS between two beings.
"This may appear to contradict the statement “I and my Father are one,” but there are two parts to the statement in recognition that the Father is greater. T-1.II.4:7
The urtext had a much fuller statement about this:
"This appears to contradict another statement: "I and my Father are one." It doesn't. There are still separate parts in the statement, in recognition of the fact that the Father is GREATER. Actually, the original statement was "are of one KIND."
In fact we are told countless times that God is GREATER than us. We are supposed to have AWE of him, as his son. That God is the FIRST cause, and we are CO-creators as secondary causes. That "I am forever an effect of God". The whole notion of equating yourself with God is nonsense because this is what the whole authority problem is about. You are NOT at the same level or role as he is.
Urtext: "Revelation is NOT reciprocal. It is always FROM God TO man. This is because God and man are NOT equal."
"God’s accomplishments are not yours, but yours are like His. ⁶He created the Sonship and you increase it. ⁷You have the power to add to the Kingdom, though not to add to the Creator of the Kingdom." T-7.I.2:5-7
There are so many aspects of ACIM that fly in the face of pure nonduality, and are contrary to the traditional idea of "oneness". It is NOT CORRECT to claim that ACIM is a non-dualistic teaching. And it is NOT correct to project an idea of "oneness" onto its statements. To be "one with" does not ERASE the fact that there are two things sharing that oneness. Heaven is all about sharing between many things, which is the opposite of many things which don't share.
In fact even individuality is an inherent part of reality. You HAD an individual existence as a soul BEFORE the separation from God ever came along. Individuality isn't even the problem. It's the unwillingness to SHARE that is the problem - selfishness is the problem. Not whether you HAVE a self.
"God, Who encompasses all being, created beings who have everything individually, but who want to share it to increase their joy.T-4.VII.5:1
Here ACIM has explicitly stated that God created a PLURAL OF BEINGS, in reality, who EACH had everything INDIVIDUALLY - as individuals, and who WANT TO SHARE so that THEIR collective joy increases. That is not in any stretch of the imagination a state of non-duality, or of "its all one thing", or "its all God".
What's needed is a much more accurate concept that maps to what ACIM actually says, and which supports and INCLUDES all of what it says, without trying to screen out stuff on the basis of making teh concept MORE right than the text. ACIM is a kind of "trans-duality", in which God has created many things which ARE still one. That the one and the many are coexisting through sharing. And that this is NOT duality, nor ANYTHING to do with the ego.
Heaven is in fact a holographic system. It is how God thinks. His whole creation and thought proceeds along holographic principles - where every part (son/creation) contains and IS the whole. This is NOT the same as duality. You have to open up to the idea that the many AND the one can COEXIST. That
the one IS many, and the many ARE one. This is stated here: "The recognition of the part as whole, and of the whole in every part is perfectly natural, for it is the way God thinks, and what is natural to Him is natural to you. T-16.II.3:3
"God has but one Son, knowing them all as One. T-9.VI.3:5 "Every child of God is one in Christ, for his being is in Christ as Christ’s is in God." T-12.VI.6:1
"God created His Sons by extending His Thought, and retaining the extensions of His Thought in His Mind. ²All His Thoughts are thus perfectly united within themselves and with each other. T-6.II.8:1-2"
Perhaps we can be willing to stop talking about ACIM as if it's some kind of non-duality teaching when it is utterly riddled with statements to the contrary. God is a creator and has a FAMILY in reality. And you are part of that family. And God LOVES YOU personally and EQUALLY.
"You cannot enter into real relationships with any of God’s Sons unless you love them all and equally. ²Love is not special. ³If you single out part of the Sonship for your love, you are imposing guilt on all your relationships and making them unreal." T-13.X.11:1-3
"Ultimately, every member of the family of God must return." T-1.V.4:1)
Btw these are ALL quotes from FIP 3rd edition. Put aside what other well-meaning people have told you and listen to what JESUS is telling you.