Final answer:
The lasting impact of Spain's use of Christianity in colonization and empire-building was the spread of Roman Catholicism and the role of Catholic missionaries in teaching and bridging the gap between the Spanish colonizers and native populations.
Step-by-step explanation:
The lasting impact of Spain's use of Christianity in colonization and empire building was the spread of Roman Catholicism, which became the largest legacy of Spain in the New World. Catholic missionaries, such as Dominican friar Bartolome de las Casas, played a significant role in teaching Christianity and bridging the gap between the Spanish colonizers and native populations. This religious conquest was brought to the New World along with the military conquest, and it shaped the colonial society and administration, as well as patterns and methods of imperial control.