How can you be half Jewish?

Judaism has evolved into being both a race and a religion. you can join the religion, but not the race.

although in judaism its commonly accepted that the mothers bloodline is all that makes you jewish by birth. so, if my information pans out, hitler was infact not a jew, but had jewish blood. i wouldnt quote me on this, but i think hitlers mother was raped by his father, who was a jewish householder.