Communication means sharing. Not sharing where one person hands over something to someone else, who now possesses that thing, but the first person doesn't. It means, you both have access to the same state at the same time concurrently. In other words, communication means Oneness.
When there is a breakdown in Oneness, where separations are introduced, it' like erecting walls that come between. Separate parts of mind - or individuals - now seem to be on either side of the wall, like the Berlin wall. This wall breaks communication. It makes it hard to share and join and connect.
Once communication has broken down, i.e. sharing has broken down, we get into a state where there is barely any real sharing. Separate individuals now seem cut off from each other with various forms of distance between them.
The physical world with all its illusions arises out of this breakdown in communication, resulting from separation and illusions of fragmentation and distance. "Spacetime" really means "distance from here."
When communication is diminished, it degrades and devolves into what we commonly think of as forms of communication. These forms of communication are severely strained because there is so much separation involved.
While you can write or speak to others, across a gap of separation there must be misinterpretation. Information will be lost, mis-perceived, and disagreed upon. When two people seem to be a little more eye-to-eye, communication may be elevated, or they may seem to share more.
But there is also an illusion of sharing. Prowess at using the ego's forms of communication doesn't necessarily mean there is openness to true sharing. There are excellent "communicators" in the world but they are not necessarily One with God, or experiencing the same exact identity and state of being as the One Son of God.
The body limits communication, and every way that we use it to express through its forms, are breakdowns in communication. Once in the body this way, unless you are so awakened that you are communicating from pure love, you will feel this urge to try to, or to have to, force others to see things the way you see them.
This happens because you know that there is a major difference between the way you perceive separately and the way they perceive separately. You have to make egoic efforts to force them to listen, and the ego then produces this fictional idea that ... if you could just get people to "get" what you are saying, if you could just get them to "see" your point of view, or if you could just shake the denial out of them to get them to agree with you wholeheartedly, THEN this would be communication. However, this is impossible to achieve.
This automatic compulsion to coerce others to see things your way, is simply because you are not both sharing the same direct experience and there is no Oneness. This means the other person will have to be changed in order for them to seem to share what you are experiencing, and this change can only be brought about through force, in defense of what you perceive as being the whole truth.
While there is a separation, there cannot be perfect communication. The ego will attempt to claim it is possible to KEEP the separation between you and others, to communicate only through its approved channels, and to nevertheless perfectly transfer what is in you to others, or to precisely get from others what is in them. This is impossible and will never work. The separation distorts the message and will always inhibit it.
True communication in ACIM means Oneness. When there is a revelation, God is communicating fully to you. When there is full awakening, you are then able to share in full communication. The Course speaks of how we are to use miracles to help us to "raise the level of communication". What this means is decreasing separation so that there can truly be real sharing - oneness.
"To be with a body is not communication."
"The whole aim of the miracle is to raise the level of communication."
"11. Prayer is the medium of miracles. It is a means of communication of the created with the Creator. Through prayer love is received, and through miracles love is expressed."
"Like any communication medium, the body receives and sends the messages that it is given. It has no feeling for them. All of the feeling with which they are invested is given by the sender and the receiver."
This also means that, in fact, the ego does not facilitate communication - it blocks it. It spins illusions of communication to try to bridge the gap of separation which it itself put in place, which is "special communication" (that's not an official course term but it could be). Illusions of successful communication, or the ways that you improve the ego's forms of communication, covers up and distorts real communication.
In truth then, nothing that the ego attempts to "share" goes anywhere. It's ultimate form of communication is attempted death, whereby the body is used as the ultimate weapon to try to prove that oneself is dead and therefore so are others. And yet, this has no effect on others. The more you enter into the ego the more cut off from others you become, and the effects of what you attempt to bring about cease all the more.
This is also why nothing real can be threatened. The ego cannot attack God. The more it attempts to attack God the less effective it is. The ego's attacks are withdrawals from God. The more it tries to bring about death, the more confined and separated it becomes, and this keeps all of its effects securely at a distance from God. The Kingdom of God cannot be assailed because the ego's forms of communication - "attack" - is actually an act of failing to will anything real.
"Exclude any part of the Kingdom from yourself and you are not whole."
"The body no more dies than it can feel. It does nothing."
"Nothing outside yourself can hurt you, or disturb your peace."
"Nothing can destroy your peace of mind because God goes with you wherever you go."
"There is no death because the Son of God is like his Father. Nothing you can do can change Eternal Love."