Full Idea
What the existentialist says is that the coward makes himself cowardly, that the hero makes himself heroic.
Gist of Idea
Existentialists says that cowards and heroes make themselves
Source
Jean-Paul Sartre (Existentialism and Humanism [1945], p.35), quoted by Christine Daigle - Jean-Paul Sartre 2.3
Book Reference
Daigle,Christine: 'Jean-Paul Sartre' [Routledge 2010], p.50
A Reaction
A nice statement of the existential plasticity of the self, in opposition to the much stronger concept of human nature in Aristotle (who nevertheless believes you can acquire virtues and vices).