ACIM is clear: No error occurred. There was no separation. Because you visually see another body, you assume (judge) that this person is different, and the difference means separation from the ego mind. The illusion of a separate identity thus becomes a reality. Now there is a specialty that frees the ego to create a God who will "give" you the special love that the other vengeful God denied during the sin of original separation. Exceptionalism also extends to your brothers, who are used by the ego to replace the love of a vengeful God.
What does it mean to see another as your brother? Let's break down the components, but be aware that even with these understandings, it will come down to your willingness to let go of judgment and do the real work of removing the ego.
Special Relationships:
These special relationships replace the love of God. They can be love or hate relationships, but either way they are based on the "wrong" or "right" ego mind. If you do one little thing that the ego misjudges, it will go from love to hate in the blink of an eye. To believe that God would do this is to believe the ego's lie that God is vengeful and attacking. You would attack others just because you believe the same lie about yourself. These are the judgments you must be willing to surrender for correction (forgiveness). Only the ego judges and attacks. You are not your ego.
Absolutely all relationships are special until you decide to make them holy. You cannot see others as your brother if you are involved in judging and projecting. The darkness blinds you to their light as well as your own. A Course In MiraclesHoly relationships are used by the Holy Spirit to correct the mind (forgiveness). This makes the choice to see another as a brother an experience that becomes real.
The only way any relationship can become a holy relationship is for at least one of you to practice forgiveness. To see someone as a brother is to do so, even if you are the only one in the relationship willing to surrender judgment. What you do as a cause will be your effect.
Speciality:
Another popular ego that prevents you from seeing another as your brother is to use your own strangeness as a way to increase separation. As long as you consider yourself better or different, you are considered special. God's special love will shine upon you because you stand out because you are unique.
Examples: Taller versus shorter. College education versus no education. Good looking versus ugly. The rich versus the poor. Healthy versus sick.
The list goes on and remember that each side is a trick because both are still ego minds. Your illness can make you as unique as your health. Remember that ego judgments are one or the other. He doesn't know any other way.
ACIM clearly points out that if you want to return home, you must let go of all judgments of others and everything in this world. As long as you judge yourself or others as special or even different, you will not see all others as brothers. Quirkiness is, by definition, separation because it makes you unique. There is only sameness and unity in heaven, and your brothers are all the same. The court hides it from you.
Forgiveness and Guilt:
The course also makes it clear that what you see in the other person is a projection of your own guilt and therefore what you need to forgive in yourself. Condemning your brother prevents you from knowing yourself directly because it distracts you from using your decision-making abilities to make amends. A tirade of judgments only shows you what you think you are: a judgmental ego. Every time you judge, you say you are. You are denying your own truth.
It's really quite simple: Look at your judgments brother. Your light and theirs is hidden in judgment. You don't see what you hide in judgment. Be willing to step back from projection and judgment and surrender them to the Holy Spirit for correction (forgiveness). This means doing the work and using the decision maker to choose the other side of the divided mind.
Surrender all judgments until you are rectified (forgiveness) and found to be true. You will see the other as a brother when the judgment about them is gone. There is no separation at this point because they are considered the same. Heaven is sameness and unity.