Christianity is usually called a religion. As a religion it has had a wider geographic spread and is more deeply rooted among more peoples than any other religion in the history of mankind.

Latourette is making a descriptive, comparative claim about Christianity’s historical diffusion: whatever one thinks of its truth-claims, it has functioned sociologically as a “religion” with an unparalleled global footprint and durability across diverse cultures. The opening—“usually called a religion”—hints at a methodological distinction common in church history: Christianity can be approached not only as doctrine or revelation but also as a historical phenomenon whose institutions, missions, and cultural adaptations can be mapped. The second sentence frames Christianity’s significance in world history in terms of breadth (geographic spread) and depth (rootedness among peoples), emphasizing long-term embeddedness rather than mere presence.