In the two opposing views of ACIM - one that God created this world, and one that he didn't, there are some parallels.
Both views recognize that the ego is a problem and seeks to undo it. Both views recognize that the Holy Spirit provides a higher vision which is more truth-oriented.
Both views also seek to see THIS world in a more loving and forgiving way.
But where they diverge, is in terms of the overall goal of the relationship with this world.
In the view that God created the Earth, forgiveness is seen as a way to love what God created, as though you were attacking a real creation through your projections of guilt, and have to learn how to forgive it and reunite with it.
In the view that God did NOT create the Earth, forgiveness is seen as a way to love what WE created, in order to see the REFLECTION of God in it, as though you were attacking a FALSE world through your projections of guilt, and have to learn how to OVERLOOK it and unite with what is BEYOND it.
Both views have us changing our relationship with the world to a more loving one. To being more forgiving towards whatever it is and whatever is in it.
The view that God created this world uses forgiveness to form a better relationship with what is thought to be God's kingdom, to not merely reflect heaven in it but to see it AS heaven literally. This view rejects the idea that heaven is another kingdom distinct from the world.
The view that God did NOT create this world essentially uses forgiveness to UNDO the world we made, where atonement is seen as bringing about the END of the world. Not ended in attack, but ended after the "benediction of holiness upon it". First forgiven, then disappeared. The ending of the world then gives way to REVEAL the underlying Kingdom of God that it was hiding.
Since A Course in Miracles has plenty of material pertaining to forgiving "a world", to being loving towards "the outside world", to no longer projecting sin and guilt onto "the world" and so on, in both models, it is easy to see that the idea of God creating the world SEEMS to have a lot of material to back it up.
But where that model falls short is revealed in the difference in its final goal. It seeks to keep the world and not have it disappear. It seeks to make the world REAL, instead of recognizing its unreality. It seeks to combine heaven with earth such that Earth is fully literally heaven. And it seeks to write-off anything suggesting that Earth is not of God, by binding it to the ego's faulty perceptions.
There is an entire step BEYOND the forgiving of this world, a step where God takes us out of this world, where the world disappears, where the stars disappear in light, where the world "merely ceases to seem to be", where it "spins into the nothingness from which it came", where space and time and all their contents disappear, and where the world indeed does come to an end. We leave bodies, we leave the world, and we go to our true home in the Kingdom of God - not of this world.
So you can see that both models have a lot of parallels, both have features that have a similar approach. A person believing that God created the Earth can still adopt a loving attitude toward the world, some FORM of forgiveness toward it, and can even approach the reflection of God - albeit interpreted as literally God itself. The reflection of heaven is "read into" as being merely a way of looking AT heaven, rather than its reflection. And so a person studying under this umbrella could still potentially reach an advanced state of some FORM of love and forgiveness. But it does not take it all the way.
Only when we can recognize that God did not create the Earth can we truly have a) a place to assign statements about Earth being not of God, b) a proper recognition of what a reflection is, c) a correct understanding of what forgiveness means and implies, d) an openness to realizing there is no Earth/world, and e) a willingness to recognize God's Kingdom as beyond this world and our true home in God.
So both approaches can take you quite far, but the belief that God created the Earth will cause you to reach an impasse. You won't be able to go INTO heaven proper, because Earth will act as a doorstop. If you think you reached your destination before you actually did, you're obviously missing out on the final goal. The road doesn't end at planet Earth, it ends in another world entirely. And so the two paths DIVERGE greatly the closer you get to the real world. One aims to settle into life on Earth forever, while the other has its sights set on a world beyond the stars.