Death

When we consider what defines humanity, there is no escaping the fact of death. The mortal body is the single thing that is guaranteed to connect all humans of any age, shape, size, status, class. It is what terrifies us in the back of the mind unless pushed to the front in a dangerous situation. It is practically the basis for "religion," at least in some way, whether conceptualizing what happens at / after death, or to the other extreme of causing, encouraging, or hoping for death of religious enemies, members, and others.