For all of the real and important contrasts between them, the Indigenous peoples and French Catholic colonists who encountered one another in 17th-century New France were both convinced that spiritual change was possible across cultural, linguistic, and ethnic lines. Both saw religious belonging and cultural identity as being essentially behavioral — and thus volitional — rather than as an immutable ethnic given. Both boasted powerful rituals that could effectively transform strangers into kin: positing a kind of symbolic rebirth or soul shift involving the reception of a new name and identity that literally and objectively re-made the individual concerned from “one of them” into “one of us.” For Indigenous people, this ritual was adoption. For Catholics, it was baptism. This presentation will explore the many fascinating parallels between Indigenous adoption and Catholic baptism in 17th-century New France, and chart how these ceremonies of transformative incorporation were themselves transformed with the imposition of foreign blood quantum measurements as an index of Indigenous identity in the late 19th century.