This morning, a Monday, the only worshippers were Mr. Takemoto, a theological student living in the mission house; Mr. Fukai, the secretary of the diocese; Mrs. Murata, the devoutly Christian housekeeper; and his fellow-priests. After Mass, while Father Kleinsorge was reading the Prayers of Thanksgiving, the siren sounded. (1.20)
It is notable that men of faith play a fairly big role in the book, and we get a lot of detail about Father Kleinsorge's religious practice/routine before the dropping of the bomb.
While he was at work, a Miss Tanaka came and said that her father had been asking for him. Mr. Tanimoto had reason to hate her father, the retired shipping-company official who, though he made a great show of his charity, was notoriously selfish and cruel, and who, just a few days before the bombing, had said openly to several people that Mr. Tanimoto was a spy for the Americans. Several times he had derided Christianity and called it anti-Japanese. […] Now he was very week and knew he was going to die. He was willing to be comforted by any religion. (3.46)
Apparently, despite the fact that he'd been a real jerk to Mr. Tanimoto and made fun of Christianity in general, Mr. Tanaka was willing to talk to pretty much anyone who could offer him any kind of spiritual comfort, when he realized he was going to die. So, he asked for Mr. Tanimoto.
Late in February, 1946, a friend of Miss Sasaki's called on Father Kleinsorge and asked him to visit her in the hospital. She had been growing more and more depressed and morbid; she seemed little interested in living. Father Kleinsorge went to see her several times. On his first visit, he kept the conversation general, formal, and yet vaguely sympathetic, and did not mention religion. Miss Sasaki herself brought it up the second time he dropped in on her. Evidently she had had some talks with a Catholic. She asked bluntly, "If your God is so good and kind, how can he let people suffer like this?" She made a gesture which took in her shrunken leg, the other patients in her room, and Hiroshima as a whole. (4.26)
Hersey describes religion/religious men having a pretty positive impact on the people around them in the wake of the bombing. When Miss Sasaki was seriously down in the dumps because of her injuries/attendant illness, a friend sent Father Kleinsorge her way. However, as you can see from this passage, he had his work cut out for him, as she was not feeling very excited about religion/God at that time.
He began thinking about raising money to restore his church in the city. He became quite friendly with Father Kleinsorge and saw the Jesuits often. He envied them their Church's wealth; they seemed to be able to do anything they wanted. He had nothing to work with except his own energy, and that was not what it had been. (4.29)
In the later chapters, we get a lot of detail about how Mr. Tanimoto's efforts to rebuild his church despite limited resources. Unfortunately, he didn't have quite the same financial support as Father Kleinsorge's Jesuits.
The Society of Jesus had been the first institution to build a relatively permanent shanty in the ruins of Hiroshima. That had been while Father Kleinsorge was in the hospital. As soon as he got back, he began living in the shack, and he and another priest, Father Laderman, who had joined him in the mission, arranged for the purchase of three of the standardized "barracks," which the city was selling at seven thousand yen apiece. They put two together, end to end, and made a pretty chapel of them; they ate in the third. (4.30)
Because of the Society of Jesuit's resources, Father Kleinsorge was ultimately able to rebuild the mission/chapel that had been destroyed in the bomb.
Whether or not Father Kleinsorge's answers to Miss Sasaki's questions about life were final and absolute truths, she seemed quickly to draw physical strength from them. Dr. Sasaki noticed it and congratulated Father Kleinsorge. By April 15th, her temperature and white count were normal and the infection in the wound was beginning to clear up. (4.31)
According to Hersey, despite Miss Sasaki's initial resistance, Father Kleinsorge's repeated visits eventually inspired a total 180 win her attitude, which contributed to a miraculous improvement in her health.
During the early summer, she prepared herself for conversion to Catholicism. In that period she had ups and downs. Her depressions were deep. (4.32)
Apparently as a result of Father Kleinsorge's influence, Miss Sasaki ended up converting to Catholicism. Remember when we said she did a 180? We weren't lying (although as you can see here, she still struggled with depression for a while).
Because he had no church into which to lure converts, if there should be any, Kiyoshi Tanimoto soon realized the futility of this evangelism (5.127).
In Hersey's 40-years-later update, we learn that Mr. Tanimoto ended up doing a bunch of speaking engagements in the States to try to make money for rebuilding his church, which also got him involved in various peacekeeping efforts/causes.
When Mother Superior, Marie Saint-Jean de Kenti, asked her one day what she would do if she were told she had failed and would have to leave, she said, "I would take hold of hat beam there and hold on with all my strength." She did hold on, and in 1957 she took vows of poverty, chastity, and obedience and became Sister Dominique Sasaki. (5.94)
Holy turnabout, Batman! Not only did Miss Sasaki end up converting to Catholicism—she ultimately became a nun. Father Kleinsorge must have been one convincing guy.
Once, an old man revealed to her on his deathbed, with such vividness she felt she was witnessing the act, that he had stabbed another man in the back and had watched him bleed to death. Though the murderer was not a Christian, Sister Sasaki told him that God forgave him, and he died in comfort. (5.98)
Although not really related to the war/any other major "plot" points, this story kind of speaks to the lessons in mercy and forgiveness that Sister Sasaki had learned over time, and also obliquely points to the lasting impact of death/traumas on the people who are involved in them, which seems kind of on-topic in a book about something like the atomic bomb…