Twentieth-century American women's religious activism did not end with the temperance movement. Nor was it limited to conservative Christianity. "The Commandment of Love" investigates the overlooked links between turn-of-the-century Christian women's reform culture and liberal social movements of the 1960s and 1970s. Case studies of two influential organizations, the Protestant Young Women's Christian Association of the USA (YWCA) and the Catholic Maryknoll Sisters, reveal an evolving body of religious ideals that were put to service in the pursuit of social change. With an understanding that the biblical injunction to love one's neighbor gave women responsibility to shape life on earth, the groups mobilized around questions of labor, international relations, and race in a decades-long quest for fellowship and world unity.

The dissertation argues that despite the perceived divide between Protestantism and Catholicism, an ethics of Christian love and dedication to single-sex community gave rise to a non-sectarian women's activism with global horizons. The organizations steadily grew more liberal and politicized, moving from an imperial evangelicalism to an interest in social justice and community empowerment. The YWCA established a breadth of programming that took a wide view of the Christian development of women. It immersed itself in reform early in its history, becoming an innovator in the labor and civil rights movements. The Maryknoll Sisters, a missionary religious order, gained new visibility for women's labor in the Catholic Church, particularly in the embrace of Cold War anti-communism. The sisters emerged from the upheavals of the 1960s as an emblem of left-wing Catholicism. At times, both groups earned acclaim for what was perceived to be selfless Christian service. At others, they came under profound censure.

Following international women's activism from the imperialistic drive of early foreign mission to the late twentieth-century pursuit of human rights, this project challenges templates of social action that privilege the secular. In a departure from recent scholarship on the rise of conservative Christian politics, it underscores the persistent significance of religion as a wellspring of liberal activism. Finally, in examining the multifaceted strategies for progressive change that came out of single-sex organizing, it documents women's rich contributions to liberal religious thought and political movements.