What is the main theme in A Farewell to Arms?

Numerous themes are developed in the novel, each
contributing to Hemingway's examination of man's essential condition, one of profound
spiritual loneliness. Through Frederic, and to a lesser degree through the other
characters, Hemingway develops the idea that life is nothing more than random, often
tragic, events, occurring without any divine plan. Various characters in the novel often
speak of God, but He is nowhere to be found; "religion" becomes an expression of
personal need and is defined, pursued, or practiced differently by the characters.
Absent God, Frederic and Catherine embrace each other. "You are my religion," Catherine
tells Frederic.

The spiritual loneliness that pervades the
novel is seen in the characters' attempts to cope with life, to either find meaning in
it or to escape it. Rinaldi operates, drinks, and sleeps with many women; the priest
prays; Count Greffi longs to become "religious" before he dies; Catherine dedicates
herself to Frederic; and Frederic faithfully performs his duties as an officer, until he
loses his ambulances and his men and is about to be shot during the insanity of war.
Hiding under a tarp on a railroad flat car, Frederic declares a "separate peace" from
the war and makes his way back to Catherine, who has become his anchor in a chaotic
world without meaning. With her death, however, he is left alone. The brutality and
futility of life is summarized in the novel's conclusion as Frederic waits for Catherine
to die:

Now
Catherine would die. that was what you did. You died. You did not know what it was
about. You never had time to learn. They threw you in and told you the rules and the
first time they caught you off base they killed you. Or they killed you gratuitously
like Aymo. Or gave you the syphilis like Rinaldi. But they killed you in the end. You
could count on that. Stay around and they would kill
you.

Following this
meditation, Frederic remembers watching ants die in a fire. He could have been their
"messiah" and saved them, yet he had watched their suffering objectively and acted only
to further their destruction. After praying in desperation to a god that does not save
Catherine, Frederic's spiritual loneliness is complete. He walks back to the hotel in
the rain, alone in the world with nothing to sustain him except his own courage. There
is no salvation; life is a condition only to be endured.