After graduating from the academy, he earned a master’s degree in computer engineering at Johns Hopkins University in preparation for what he said he expected to be a career in nuclear submarines.

But Mr. Izbicki said he also began exploring his commitment to Christianity. He studied the Gospels, read widely about the early history of the church, took up Hebrew so he could read the Old Testament in the original, and started to measure his faith according to the evangelical touchstone “What would Jesus do?”

It was in that light that he encountered the exam question about launching a nuclear missile in early 2009, shortly after he was assigned to submariner school at the Nuclear Power Training Command in Charleston, S.C. Seeing the question spelled out like that, he said, made it impossible to hide his emerging pacifism any longer.

“I realized that I could not be responsible for killing anyone,” he later explained.