Frederic Henry, an American ambulance driver and a lieutenant (“tenente”) in the Italian army, is the narrator and protagonist of the novel. Henry is characterized initially by a sort of detachment from life-though well-disciplined and friendly, he feels as if he has nothing to do with the war. These feelings of detachment are pushed away when Henry falls in love with Catherine and begins to realize the hostile nature of the world. In this way, Henry serves the function of a character who becomes initiated in Hemingway’s existentialist philosophy of an indifferent universe and man’s struggle against it. Existentialism is a philosophical movement that claims that individual human beings have full responsibility for creating the meanings of their own lives.

This calls for many things, the first of which being a disbelief in God-to Hemingway, such faith was a cheap way of falsely instilling order upon existence (this is where the priest falls short). Because there is no God, there are no universal moral codes, no abstract values such as “justice” or “glory,” and certainly no need for moral conventions. The code hero rejects these, but imposes order upon his life through personal values-integrity, dignity, courage, etc. This is what Catherine knows from the beginning and Henry learns in the course of the war. In essence, the hero learns that he, himself, is a crucial source of meaning. Finally, such a person must accept the finality of death, knowing himself to be caught in a meaningless existence.

SARTRE:

Jean-Paul Sartre graduated from the École Normale Supérieure in 1929 with a doctorate in philosophy and served as a conscript in the French Army from 1929 to 1931. He founded the philosophy of existentialism. In 1939 Sartre was drafted into the French army, where he served as a meteorologist. German troops captured him in 1940 in Padoux, and he spent nine months as a prisoner of war — in Nancy and finally in Stalag 12D, Trier, where he wrote his first theatrical piece

Read his biography on wikipedia. There are plenty of parallels