"I cannot see in darkness but in light"
"God is the light in which I see."
"Eternity will shine away the world."
"The world will disappear in light."
In ACIM there is a theme of the relationship of light and dark. We're simply told light dispels darkness, and that really there is no such thing as darkness. Darkness is a label for the illusion of a "lack of light". Essentially this means there is either light or a lack of light, and darkness isn't "a real thing". It's similar to saying there is love or a lack of love, but there isn't real evil. This is forgiving because it recognizes either love or a call for love and doesn't regard the "lack of love" as a state that's real or "actually evil."
That said, when we dreamed of death and made a world, which includes Earth, and all bodies, we made these things by pretending there is a lack of light. And this absence of light seems to show up as darkness, as if to say it is "made of darkness". As soon as you switch from realizing there is only light or its lack, to believing that the lack of light is itself something which can CAUSE light to go away, you start to believe that the darkness is real and that "darkness can hide." It's the belief that darkness has power, is evil, is real, and is capable of forcing light to be dispelled. It's nothing other than the mistaken belief in sin - a dark thought being able to attack the light of God. It's a level confused idea.
In truth, since there is only light or its illusion of absence, when light is supplied, the illusion of the lack of light disappears. Miracles supply a lack. Once the "lack of light" is overcome by supplying the light, and the light is present, there simply is no lack of light any more and so "the darkness" simply vanishes. In the presence of light there is no darkness. That which seemed to be "made of darkness" is really just light's seeming absence, and now that light has arrived, that which was "made of darkness" must be shone away and disappear. It really was only an illusion of darkness seeming to exist, when in fact it was nothing. Darkness isn't real.
For example, when clouds block the sun from shining, and the sun is still shining beyond the clouds, the underside of the clouds seem to become dark because it now seems as if the clouds have the power to block the light. The clouds are symbolic of the idea that darkness has power over the light and can prevent light from extending. Blocks to awareness do the same thing. But the fact is that the clouds only become dark because of the seeming illusion of an absence of light, not because clouds are inherently dark. They are made of transparent water. When light shines upon them, they are not dark at all. They are seen IN the light WITH light and this light thus dispels the belief that "the clouds are dark". Darkness is an illusion.
Similarly with shadows, a shadow is merely the illusion of an absence of light. When light shines onto the area which was "in shadow", the shadow disappears. Light always has power over darkness EVEN in this dream world. Whenever the light comes, darkness simply vanishes and whatever evil figures seemed to be made of darkness, absolutely cease to be. It has no protest, it has no power, it has no way to prevent itself from being shone away. Darkness has no defense against the light. It simply disappears because it wasn't really a thing in its own right, it was just the absence of light. Once the absence of God is supplied, once love is given to those who lack the love, the absence of love is restored to love and all seeming illusions and manifestations of darkness simply disappear. Light corrects the darkness by showing that there is no darkness, just as atonement shows there is no sin.
Light has power over darkness because there is no REAL opposite to light. Darkness is not reality. Light is reality. Darkness, as the opposite to light, is just an illusion of darkness and so the light always has power over it. Light is permanently true and unchangeable. Darkness is a temporary illusion that what is permanent is not permanent. Darkness is a lie, a deception. It doesn't really exist.
The world, which we made, with bodies, are all made out of darkness. They are the "absence of light" attempting to be maintained by minds that are trying to believe light is not present. And so they are trying withhold their light in order to stop the shining in order to make it seem like there is a "hole" in the light where light has become absent, where heaven has turned into hell. And then it becomes easy to believe, the seeming lack of light - which is showing up as darkness - is actually a "real darkness", as if the darkness itself is a thing in its own right, with a power and a will and a causality and capable of causing light to go away. Like since when did darkness ever have the ability to resist light? But while we believe there is a world here, we are believing that darkness has power over light. We're looking at nothing but an illusion of light's seeming absence, not real darkness. When light is allowed to shine and enter, light's absence is corrected and there is now no darkness for the world to be made of.
Given also that the world and bodies are all "made of darkness" or are the illusion of light's absence, when the light comes, those things will be shone away. When we are no longer interested in making illusions or playing with this idea that we can cause the light to be harmed or affected and caused to leave by our dark powers, we will simply allow light to shine and it WILL shine away the world and the body. They must be shone away because they are ONLY the illusion of the absence of light. When the light comes, the world has to disappear. All shadows must disappear. The body must disappear. They cannot coexist with the light because they are founded entirely on light's absence. Any more than the ego, which is darkness, can go into the light and keep its darkness. This is why that which is "made of darkness" cannot enter the light or have any ability to attack God's Kingdom.
This is also why "a forgiven world cannot last." If you forgive the world you are shining a light ONTO the world, illuminating it, and thereby SHINING IT AWAY, because it is made of light's absence and when the light comes the absence is corrected and undone, light replaces darkness, and so the world which was the illusion of light being absent, is shone away and undone. This is why ACIM says that "the world will disappear in light" and "the world will spin into the nothingness from which it came". You cannot forgive a world, shine light onto it, see it in light, and expect it to stay IF it is not made of light. And this world, Earth, is not made of light - it is the idea of sin which is the idea of darkness being able to hide or block the light of Heaven. As soon as there is light, there is no world. This is also why there are no worlds or bodies in Heaven - darkness does not exist there and these things are completely "made of" darkness. The illusion that there is no light. They were made to block the light. They cannot enter or be in the light and maintain their state of darkness, thus they cannot exist at all.
This is why eternity will eventually shine away the world. The world, its forms, its bodies, which are all lack of light, will become filled with light so much that the light will dispel the lack of light, and so all of the forms will be shone away entirely. They will leave absolutely no trace in their going because they were always illusions of darkness or light's absence. Temporarily, the world may seem to become a shining reflection of Heaven or to be so filled with light ("I am the light of the world") that it seems heaven-like... but only temporarily, in a transition from dark to light, because when enough light has shone upon it, it will be so infused with light its darkness simply MUST vanish. When the light becomes brilliant, intense, powerful and full, there is nowhere for darkness to exist or hide, and all things "made of darkness" will vanish as if they never existed. The world will be healed away.
So long as forms and objects are maintained, there is a protest against light and an attempt to block it. Physical objects are blocks to the light just as they are blocks to awareness. They are made of the idea of sin, the suggestion that darkness can block the light and have power over the light. This is why "to see a sinless body is impossible" - you cannot look upon sin unless you are sinful and when you are purely innocent you can see nothing but light. And this is why you cannot truly be 100% in the light of heaven, while having a body.
The light (of the great rays for example) will literally fill the void where the body was pretending to exist, and replace it, filling up the gap with light, since the body is nothing but the illusion that there is a lack of light or a block to light. The body is what a lack of light "looks like", something dark and an obstacle to light seemingly capable of obscuring it. And since the body blocks light, it also blocks the mind and awareness of truth and atonement. "The body is a limit" "The body cannot stay." So long as the body is present at all, it is blocking light or demonstrating that darkness still exists.
There are no obstacles to light in the living world of light, which is Heaven. There are no bodies, forms, objects, walls, or anything that can keep light from being absolutely infinite and intensely brilliant everywhere. There is nowhere "lacking light" for the body to seem to exist. There is only light everywhere.
"Nothing there but shines and shines forever."
"Only the creations of light are real."