The first thing is this.
Jesus wrote ACIM and he is not in a body, and throughout the Course we're taught we are not a body. That's even today's workbook lesson - I am not a body I am free. That doesn't mean, oh, you're still IN a body but it's not you. Also the course says at no time does the body exist. Ok. So.... WHERE do you exist? Where is there no physical matter? Not this universe.
Clearly this means there is some other seeming 'place', if you will, in which mind - spirit - is able to exist, which does not require a physical form and has nothing to do with the physical world, which, by the way, is not real.
So here we at the very least have 2 `levels`. The level of the physical world of form, and some other spiritual level. Sort of like a 'dimension' if you will. It's non-physical. It's where you end up when your body dies. And its the stopping off point between lifetimes.
Now, each 'level' has to be like a 'place' of sorts.. which the Course refers to as a 'universe'. And if you read the text carefully, when it mentions universe it uses that exact same word even though it's sometimes referring to the universe of form, and sometimes referring to the universe that God created. ie reference to at least some other non-physical level or experiential existence.
So if you don't believe there are levels, and you read the course talking about 'universe' in this way, and you don't discern exactly what that relates to given the context, then you're going to assume it's always talking about the physical universe. And then when it makes statements about 'universe' it's going to throw you into a whole bunch of confusing misunderstandings and ego interpretations. You'll attribute all kinds of 'truth' to the ego universe because you think the course is telling you this is the case.
The course is often quite subtle about this, and it also shifts around quite a lot between which 'level' of experience it's referring to, without necessarily giving you a nice obvious disclaimer that it's doing so.
Now, this other universe, the spiritual universe, also has to be broken down into sub-levels because there are parts of the course which speak of certain truths which cannot possibly apply to a separate place like that. For example whenever the Course speaks of 'heaven', and of 'souls', and of 'Gods Creation', and of 'knowing', and 'oneness' and all these interesting things, they often refer to the very highest 'level' of the metaphysics - heaven if you will.
But then at other times the course speaks of things like the sonship, alluding to you having multiple brothers, a brotherhood, not in terms of all brothers being ONE SON, but in terms of a multitude of sons. Also the urtext speaks of multiple souls. Now, in terms of the metaphysics, it does not make logical consistent sence to group together references to these 'sub-level' ways of looking at things with the highest-level way of looking at things. If you do that, then when the course refers to something like heaven or knowing or whatever, or the son of God, you will lump it all together under a general umbrella of 'heavenly spiritual stuff' and make the only distinction that this apparently is 'not the ego' level. This will result in all manner of confusions, misunderstandings of meaning, wrong implications, false conclusions and just a big mess. The metaphysics will break, the course will become inconsistent and you won't be able to understand it.
So now at the very least, there is a level of heaven, a level where the sonship exists as multiple sons, a level of christ where there is only one son, and a level of the physical universe where the ego plays out in duality. This is all quite clearly in the course text throughout and referred to in multiple ways and in multiple contexts. And if you read it and if you understand what it means, then when you read about christ you know that its referring to the unified one son and when its referring to sonship or brotherhood you know its referring to the level of multiple sons.
So there is a level of the metaphysics, which has its own 'universe' and its own 'state of mind', like a 'place' of experience if you will, for each of these aspects of the course. This is the structure of the dream that the ego made. Different levels of 'splits' in the one mind. And these are on a colossal scale. Astronomical. The ego dream comprises separation from God, resulting in a separate son 'level', and then a split into a sonship/brotherhood which is a whole other universe/level, and then there's the level just above the physical (level 2) where the holy spirit resides, and then the physical level. All of these levels are explicitly referred to in the course and if you really can grasp the logical and subtle implications of every thing Jesus says in each section, you can tell that these levels 'exist' in the metaphysical structure.
So now, if you don't believe in these levels, then as mentioned, you will draw all kinds of false conclusions. It results in all kinds of behaviors where you try to attain states of mind/being/experience which are extremely far beyond the 'level' of the dream that you find yourself on. You generally can only transition to the next highest level, one level at a time, you can't easily jump to the top - to heaven. Holy Spirit is on level '2', in a different 'dimension' if you will, which is where we all are being asked to get to as our main next step. When we try to move beyond that to things like awareness of the sonship, total equality, oneness, christ-mind, etc... we basically CANNOT do it and it leads us into all kinds of ego deceptions - a spiritualized ego.
The spiritualized ego is what happens when the ego, or shall we say the mindset, of a given 'level 'of the ego's total dream, attempts to transfer itself to a higher-order level of existence. It tries to KEEP its mindset (e.g. level 1 ego duality) but at the same time take itself to the next level where there is really a totally different mind (Holy Spirit's mind). But this is impossible, of course, but this doesn't stop it trying. And this creates a great amount of specialness and fake spirituality and fake progress and ego images and behavioral fuckups which call for sacrifice and illusions and all kinds of madness. It's not possible. We have to let the 'mindset' of level 1 - the ego - we have to let it GO, detach from it, so that who WE are can travel up to level 2 and proceed from there.
So anyway... if there's only one level, or even only 2 levels, you will develop a spiritual ego from it because it will lead you to so many false conclusions and so many attempts to greatly spiritualize lower-order levels of existence that cannot contain those truths. The lower mind cannot grasp and experience the higher states.
If you read the Course with these levels in mind, it is much more clear where and when Jesus is referring to certain specific levels of the dream - different dimensions, different universes, different mindsets and different aspects of the metaphysics. Also if you look at your own 'course attitude' based on these levels, you will see that you've been attempting to become an achiever of very high-level metaphysics but still while being a level-1 identity/mind. This produces bliss-ninny spiritualization of the ego, and it is rife in the course community. And in myself as well. I'm just starting to realize how far this has gone.
So anyway. The levels are just a way to map actual levels of experiential existence in the hierarchy of the total dream, which comprises several 'dimensions' of which the physical universe is only one, and therefore it's really just a clear and truthful acknowledgement of this fact, allowing you to navigate realistically and to understand what the heck the course really means when it says something. Very high-level truths are of NO USE TO YOU when you are on level 1. You cannot be christ when you are a level-1 ego. You have to become level 2, then 3, then 4, then 5. It is also a relief to let go of the pressure of trying to attain such impossibly high spiritual states which are out of reach of the level-1 mind. You can attain them but your ego cannot, and until you let go of 'level 1' thinking you cannot get to level 2 yet alone up to heaven.