The foundation for inner peace offers this on it’s website to introduce and explain A Course in Miracles. 

A Course in Miracles is a complete self-study spiritual thought system. As a three-volume curriculum consisting of a Text, Workbook for Students, and Manual for Teachers, it teaches that the way to universal love and peace—or remembering God—is by undoing guilt through forgiving others. The Course thus focuses on the healing of relationships and making them holy. 
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A Course in Miracles also emphasizes that it is but one version of the universal curriculum, of which there are “many thousands.” Consequently, even though the language of the Course is that of traditional Christianity, it expresses a non-sectarian, non-denominational spirituality. A Course in Miracles therefore is a universal spiritual teaching, not a religion 

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A Course in Miracles was scribed by Dr. Helen Schucman through a process of inner dictation she identified as coming from Jesus. A clinical and research psychologist and tenured Associate Professor of Medical Psychology, she was assisted by Dr. William Thetford, her department head, who was also a tenured Professor of Medical Psychology at the Medical Center where they both worked.

Jon Mundy of Miracles in Manhattan, was a contemporary of Helen Schuman and says A Course in Miracles teaches to: “Do nothing about nothing”. If I were to explain Jon’s words the Course says the world we live in is a result of the ego, a fake world in conflict with God and Heaven and as such it does not exist. So instead of reacting strongly to this world we see, we need to learn to forgive it or . . . to “do nothing about nothing.” 

In my own words if I were to summarize the teachings of A Course in Miracles then it asks us to Forgive EVERYONE (we have ever met, read about, heard about or in any way encountered) for EVERYTHING (anything and everything that comes to mind) ALWAYS (in all situations for all time).