Some stuff I've read talks about the soul is shaped like a Merkabah, which is also shaped like the star of David, only it's 3 dimensional and the points stick out all around. I think they got their symbol from there. As a side note, the swastika shape was used by native Americans from 1890 until about 1940; I think the nazi's stole it from them. Here is a wikipedia link: https://en.wikipedia.org/wiki/Western_use_of_the_swastika_in_the_early_20th_century