One hundred and forty-one people from MacKay Presbyterian Church, in Ottawa, served in the First World War. This is an astonishing record, but one that was by no means uncommon in Canada. Why did these men, their families, and their church enlist in this great war for “justice, truth, and righteousness, and for the Glory of God”? What was the impact of war on the surviving soldiers as they and their families adjusted to a changed world, to permanent injuries and to painful memories? This study of the experience of one church at war weaves together the stories of soldiers on the battlefields of Europe with those of the families who waited and prayed, enduring privation, fear, loneliness, and grief. It centres on the 19 men who fell in the war — some as heroes in desperate battles, others with tragic randomness or from illness, several with no known graves — and the widows they left to cope as best they could, the children who grew up without fathers, and the families who mourned their loss even as they took pride in their sacrifice. Using new methods including online research and the tools of genealogical study to bring to life people who did not leave a rich legacy of information on their lives and families, this study of a church at war deepens our understanding of the social history of Canada’s participation in the First World War, and provides a model for research on churches, communities, and institutions.