What is the difference between Docetism and Manichaenism concerning the Incarnation? I was doing a chapter in the 7th grade Faith and Life Religious Ed books with my students and there was a part about "errors concerning the Incarnation".  It covered Arianism and Docetism and the part about Docetism, which I'd never heard about before, sounded a lot like what I knew about Manichaenism.  I told the kids, since we're from St. Augustine's Parish, that St. Augustine believed all that stuff before his conversion.  But I'm not sure if that was right.

Concerning who Jesus was, it seems that both heresies taught the same thing.  That He was only God and not really a man. And both are reduced to the same fundamental body=bad soul=good problem.  So, what is the difference between the two?  Are Docetists always Christian and the followers of Mani pantheists?
Docetism can best be thought of as one of the Christological heresies into which Manichaenism falls.

Docetism is very specific Christological heresy.  It is a Gnostic idea that Jesus was never born, nor was he ever matter.  Gnostics believed that all matter was inherently evil, and therefore it was anathema to them that God could become matter.  In order to avoid this distasteful idea, the Gnostics presupposed that Jesus was merely a "phantasm" an apparation of light that looked like a person.  In other words, Jesus only appeared to be a man, which is what docetism means.

Manus, a Persian Philosopher had an entire (if heretical) belief system which included several gnostic ideas.  Like the Gnostics, he too believed that matter was evil and therefore God could not be born or incarnate in material form.  As such, it is most proper to say that the philosophy / religion of Manichaenism includes a docetic Christology.