What God creates must be permanent because He is permanence and can only create like Himself.
Therefore, if something is not permanent, if it changes at all in any way, if it is born or dies, or if it comes and goes, or transforms, or isn't always the same forever, then God did not create it.
Since God is reality, being the consistency and constancy needed for there to BE a "reality", some state which never wavers and is completely reliable, then anything that wavers or changes is not REAL.
Therefore, the physical world, the physical universe, everything in it, all bodies, everything you see here, and all forms of perceiving it, cannot be real.
Since what is real actually exists, permanently, and since what is unreal cannot possibly exist because it is not permanent and always changes, what changes cannot be REAL.
Therefore, because the physical universe is constantly changing, it CANNOT be real, and it MUST be an illusion. God cannot have created it, and it cannot really exist.
It is as plain and simple as this.
God is infinite. Infinite means in-finite, not finite. Finite means limitations, and limitations allow multiple possibilities. Infinity does not mean 'all possibilities'. It means all REAL possibility, but in terms of number, there is only ONE possibility. Infinity means oneness - one possibility, one reality, one way. This is how God is infinite. Infinite does not mean that God creates all kinds of forms and many multitudes of different things. God only creates ONE state of being and nothing else.
Nothing real can be threatened, because nothing unreal exists. God cannot be threatened because the physical world and all of its weapons of separation does not exist. This world is nothing but an ILLUSION that a world exists, and if you believe it and perceive it then you have fallen for the illusion and are not awake.
"There is no world." This is the central thought the course would aim to teach. - ACIM