There are over 250 references to this in the course explicitly tying God to the creations of many individuals. To refuse to accept this is simply unwillingness and denial. It means that oneness does not equate to "no individuals". It also means ACIM is NOT non-duality and we are not one big blob.
The information below is courtesy of Robert Perry.
The Course uses a number of terms to speak of the plurality of Sons.
Sons
The Course uses the term "Sons" 82 times. Simply using a plural term like this, of course, implies that there is more than one Son. Further, the Sons are often associated directly with Heaven, implying that they are not just post-separation illusions: "The Kingdom of God includes all His Sons" (T-7.XI.7:10).
Children
The Course uses the term "children" to refer to the "children of God" 42 times. This, of course, is another plural term. It, too, is often used to speak of God's children in Heaven, not just on earth, thus making clear that it's not just a post-separation concept. For example, "God knows His children with perfect certainty. He created them by knowing them" (T-3.III.7:9-10).
Creations
Creation is a technical term in the Course, referring to an act that takes place solely in Heaven. The Course is clear that no creation takes place in this world: "In this world it is impossible to create" (T-17.IV.2:1). Interestingly, the Course uses the plural word "creations" 118 times, mostly referring to God's creations (though many times to our own heavenly creations). For instance, "In the creation, God extended Himself to His creations and imbued them with the same loving Will to create" (T-2.I.1:2).
Sonship
The term "Sonship" is used 104 times. It is particularly significant because it at once captures both the unity and plurality of the Son. Merriam-Webster Dictionary defines the suffix "-ship" as referring to "the body of persons participating in a specified activity." The Sonship, then, is "the body of persons participating in the activity of being God's Son." It's clearly a membership concept. The Course refers to this, talking about "the members of the Sonship" (T-1.II.6:2). Further, the Course clearly links the Sonship with creation, which rules out the idea that the Sonship is a post-separation product: "The Sonship is the sum of all that God created" (T-1.I.19:2). "He [God] created the Sonship" (T-7.I.2:6).
Other terms
There are a number of other terms the Course uses to denote the plurality of Sons. Parts: "the parts of God's Son" (T-20.V.1:5), "parts of the Sonship" (T-12.IV.6:8). Aspects: "all aspects of the Sonship" (T-13.VI.6:4). Extensions: "His [God's] extensions are like Him" (T-12.IV.6:8). Channels: "His [God's] communication channels are not open to Him" (T-6.V.1:5). Brothers: "In every case you have thought wrongly about some brother God created" (T-4.IV.2:3). Minds: "all the minds which God created one with me" (W-pI.156.8:6). God's Thoughts: "All His Thoughts are thus perfectly united within themselves and with each other" (T-6.II.8:1-2). Souls: This term was used copiously in the original dictation of the Course, and eleven references to it remain. It, of course, captures exactly the notion we are talking about, that of a God-created spiritual identity that is in some sense individual.
"The body cannot create, and the belief that it can, a fundamental error, produces all physical symptoms. Physical illness represents a belief in magic. The whole distortion that made magic rests on the belief that there is a creative ability in matter which the mind cannot control." T-2.IV.2:6-8.
Another way to put this, is the belief that physical matter, form, body, world, is capable of CAUSING something to happen.
Elsewhere I recall, I think in the early urtext, Jesus telling Helen that anxiety must arise when you believe that illusions have the power to do something to you. Thus the belief in magic is the belief that stuff in the world is not only causing bad things to happen to you, but that you can use the same magic to undo that magic.
It should be obvious how self-contradictory that is. Trying to use a magical power outside of you in order to counteract the effects of a magical power outside of you, means that you will just continue to reinforce the belief in magic by trying to use it against itself.
Believing that the body or the world has a mind of its own is a belief in magic. It's believing that somehow the output or effects of the system have gained some kind of magical will of their own, that they've taken over, they've gone rogue and are now running the show. As though images coming out of a projector come alive on the screen and start directing their own movie.
Jesus is suggesting that believing in magic will 'cause sickness'. Ie, if you believe the world or body can affect you, that GIVES power to it and manipulates it as though it magically is doing something to you on its own. That is self attack, and it will produce symptoms of sickness.
If a person has even a tiny amount of unawareness, they are in a state of believing in magic.
Perceiving that you did not choose what has happened, and that therefore there is some other 'will 'or 'cause' separate from you, is a belief in magical causes. Perceiving that other people do things to you against your will is a belief in magic.
Magic essentially is victim consciousness.
The ego does not exist. It cannot be loved or embraced. It cannot be turned into a friend. It cannot wake up or heal or convert into something spiritual. It is nothing but an artificial idea of impossibility, and as such is impossible. It does not exist, cannot exist, and has no reality. To have a relationship with it is it attempting to have a relationship with itself.
Light dispels darkness. Ego is darkness. Love is light. If you shine your love in the direction of the ego darkness, it simply will disappear, because it does not exist as anything other than the illusion of the absence of love, which is impossible. It's not there. Shadows are not truth, light is truth. And darkness cannot hide the light. You cannot "love your ego" without your love disappearing it.
While it may be a stepping stone to learn not to fight with the ego, since that is the ego fighting itself, and to not resist it, and to not attack the body, and to undo all basis for ego thinking and reactions "to" the ego, this is only so that the ego itself is undone. The ego is already undone, we just are hallucinating that it exists at all. It never existed to begin with.
Eventually we have to look within and be willing to see that not only is there no sin, there is no ego either. There IS no central core of isolation. As the clouds which promised to conceal an underlying artificial identity clear, you discover that there is nothing there. It was not what you expected. There is no ego to go up against or even to really undo. Enlightenment is a recognition and not a change at all.
Holy Spirit knows that the ego does not exist and this does this forgiveness teach:
"T-9.IV.5. Forgiveness that is learned of me does not use fear to undo fear."
(do not use fearfulness or making illusions real in an attempt to relate to or deal with or undo or address the ego at all)
"2 Nor does it make real the unreal and then destroy it."
(do not make the ego a real thing and then attempt to have a relationship with it or deal with it or change it or do anything really about it directly)
"3 Forgiveness through the Holy Spirit lies simply in looking beyond error from the beginning, and thus keeping it unreal for you."
(True forgiveness is fore-giveness, it gives ahead of time. That means, it looks WITH a forgiving attitude, IN ORDER TO ENSURE that it is not mistaken, so that it does not MAKE error or ego, so that there IS no ego or error to have to undo. Only when forgiveness fails do we enter judgement - he who would not forgive must judge - which makes the ego, and so if we simply remain fore-giving the ego does not come into manifestation at all.)
"4 Do not let any belief in its realness enter your mind, or you will also believe that you must undo what you have made in order to be forgiven."
(Pretty straightforward isn't it? If you make the ego real in any way then you will start to fight with it and believe that you have work to do or stuff to fix or whatever, and doing so you will be stuck.)
"5 What has no effect does not exist, and to the Holy Spirit the effects of error are nonexistent."
(Error does not exist because true forgiveness fore-gives, failing to make errors or perceive erroneously, and therefore RECOGNIZES that there IS no error, therefore there is no ego.)
"6 By steadily and consistently cancelling out all its effects, everywhere and in all respects, He teaches that the ego does not exist and proves it."
(There is no ego, period. Believing there is an ego, is an attempt to make an ego real, which is impossible. Ego is fiction. It's just a useful concept for teaching that it does not exist, and nothing else.)