A Farewell to Arms is viewed primarily as a war novel or as a love story, Hemingway's Romeo and Juliet, but religion plays a very significant role in it. A good thesis, and one that would be easily supported from the text, would be to state that religion is a major motif in the novel.
Internal research shows that the major characters, and the minor characters, as well, bring numerous discussions of religion into the story at various times and in differing situations. Frederic discusses religion with several characters, including Catherine, who tells him that he is her religion. On one occasion, Frederic draws a distinction between being religious and being like Jesus. Count Greffi and Frederic have an extended discussion about religious faith; the Count reminds Frederic that love is "a religious feeling," and both he and Frederic hope to eventually find faith in God. The priest speaks often of religion, and when Passini is horribly and mortally wounded in battle, his suffering is expressed in terms of religion. As Catherine lays dying, Frederic prays desperately for her to live.
Taking notes over every religious reference in the novel would make it possible to categorize them in some way so as to develop a working outline to support the thesis. Some possible outline divisions might be conventional expressions of religion, unconventional expressions of religion, the search for religious faith, the rejection of religion, and religion born of suffering, to think of only a few.