If a work of art is rich and vital and complete, those who have artistic instincts will see its beauty, and those to whom ethics appeal more strongly than aesthetics will see its moral lesson. It will fill the cowardly with terror, and the unclean will see in it their own shame.

This quote is saying that when a work of art is well made, with all its elements in it, people will be able to appreciate it in different ways. People who instinctively understand art will be able to see its beauty. People who are interested in ethical issues will see the moral message. Others may be scared by it while those who have been wrong will be ashamed.