In terms of characters and characterization (versus plot and theme), A Farewell to Arms is the story of Lieutenant Frederic Henry and the way he grows and changes, lives and learns, in order to catch up to the Nurse Catherine Barkley with respect to experience and the wisdom that it brings. Especially considering that Ernest Hemingway has been accused of misogyny, it is fascinating to note that Catherine is the more mature of the two characters when they meet; therefore, it is Henry who must struggle to match her level of maturity.
Returning from his leave near the start of the novel, Henry knows he should have traveled to the priest's home region of Abruzzi, a "place where the roads were frozen and hard as iron, where it was clear and cold and dry and the snow was dry and powdery and hare-tracks in the snow and the peasants took off their hats and called you Lord and there was good hunting." Instead he has visited bars and whorehouses in the cities of the lowlands. For now, Henry's strategy vis-à-vis the war specifically and the unpleasantness of the world in general, could be referred to as obliteration, which he achieves via alcohol and sex. He is spiritually lost when we meet him, and A Farewell to Arms will trace his movement toward an understanding of the world and of himself.
It becomes apparent as soon as they meet that Catherine is different — more mature, in a word — and the characters' contrasting levels of maturity are demonstrated by their different attitudes toward the war. Henry suggests "Let's drop the war." With her characteristic mix of wisdom and humor, Catherine replies, "It's very hard. There's no place to drop it." Permanently scarred by the loss of her fiancé, she already knows that the war can't simply be "dropped."
Additionally, Henry tells us that his declaration of love for Catherine is a lie. "I did not love Catherine Barkley nor had any idea of loving her," he elaborates, comparing their affair to a bridge game. He seems almost boyish at this point in the story, and in a way that isn't necessarily appealing or admirable.
Remember, however, that Henry does not participate in the tormenting of the priest in his unit. He perhaps recognizes that the chaplain stands for something, unlike the cynical, nihilistic officers who taunt him. Henry himself doesn't believe in much of anything yet, but his refusal to join in the ritual of priest-baiting shows us that he respects those who do and that he has potential in this regard. Still, he is uncommitted enough at this point in the story that he can miss an evening with Catherine altogether because he is drinking with his fellow officers. On the other hand, he does regret doing so afterwards. Henry is growing, and growing closer to Catherine.
Just prior to receiving his war wound, Henry is still talking abstractly about bravery (though, significantly, he does admit after the first shelling to being scared). In a bit of foreshadowing that will prove ironic, he argues against giving up: "It would only be worse if we stopped fighting." He says that defeat is worse than war itself. As a result of his own intense pain, however, and the trauma of witnessing of the death of a comrade (not to mention the scene in which he is soaked by the blood of a dying soldier), Henry will no longer be able to deny his involvement in this war or its potential to affect him. He has therefore grown closer to Catherine.
While visiting Henry in the field hospital, the priest tells Henry that the war is made by certain people and executed by others. Henry still resists this notion. He also admits that he does not love God — that perhaps he does not love anyone. "You will," the priest reassures him. "I know you will." Clearly the priest knows Henry better than Henry knows himself. Note the particular nature of the contrast between the peace-loving priest and Henry's roommate Rinaldi, who is warm and likeable but attracted by the violence and sex associated with wartime. At this point in A Farewell to Arms, Henry stands somewhere between them, philosophically, as if at a crossroads. It is unclear whose path he will follow, despite his traumatic and painful recent experience.
When Henry and Catherine reunite in Milan, he again declares his love for her — only this time he means it. For Henry, his affair with Catherine is no longer a game, and it is significant that this transformation follows his wounding in battle. The experience has matured Henry, elevating him to a level of wisdom closer to that of Catherine. And yet the dynamic of Henry's naivete versus Catherine's experience and maturity is reiterated as Henry tries to make a date for the night after the operation, and she insists he will be in no shape to see her.
At last Henry's character changes fundamentally during the course of the summer he spends with Catherine; on the heels of his traumatic experience at the front, a love affair with a woman (rather than mere sex with prostitutes) forces him to grow up for good. This change is demonstrated at the start of Chapter XXXIV, after Henry's desertion from the Italian army. Of the hostile aviators with whom he shares a train compartment, he says that "in the old days I would have insulted them and picked a fight." Now, no longer insecure due to his experiences in love and war, he does not even feel insulted. In fact, as his talk with Count Greffi reveals, the once-indifferent Henry has truly found something to believe in. He tells the Count that what he values most is someone he loves and that he "might become very devout," elaborating that his religious feeling comes at night. Like Catherine, Henry has made a religion of their love. For that matter, he has replaced his loyalty to the Italian army with loyalty to Catherine.
In Switzerland, Catherine suggests she and Henry wear their hair the same length, so as to be more alike. "Oh darling," she says, "I want you so much I want to be you too." Henry replies, "You are. We're the same one." And regarding experience and the maturity it yields, he is right. At last Frederic Henry has drawn abreast of Catherine Barkley with respect to wisdom about the world. How has he done so? By participating in love and war, and by making the hard choices that both demand.
When he walks out of the hospital at novel's end, Lieutenant Frederic Henry is a different man than he was at the opening of A Farewell to Arms. He has caught up to Catherine Barkley and now understands the world and his place in it. Sadly, he carries that understanding into the rain alone and broken, and forever without her.