The Holy Spirit is a decision and the ego is a decision. One is a decision to forget who you are and the other is a decision to remember. This world was made by the ego as an attempt to keep them both active as if good and evil can coexist. It is better to see it as the mind vacillating back and forth between the two decisions.
Decisions are continuous but they are not fully in awareness. For example, people are not usually aware that an illness is the result of a decision. Instead, we tend to think there is some cause in the world, e.g., sunburn, viruses, genetics. It all seems very scientific and proven. This requires a look at the whole topic of causation. The Course Workbook lessons invite you to look at what you perceive and believe, which is necessary for the truth to be recognized. Jesus says that the world is full of spurious cause and effect relationships. Therefore, his Course works with the mind to loosen it from false beliefs, all of which were invented by the ego to reinforce the belief that we have separated from God and are guilty for that.
The whole system of linear time is based on the idea that you were guilty in the past and will be punished in the future. The mind training of ACIM is the means given to escape from this. It helps us take the power back by realizing that, as it teaches in Lesson 281, “I can be hurt by nothing but my thoughts.” W-281