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.