Final answer:
The Christian Basilica was the Roman engineering marvel that allowed missionaries to establish new churches throughout the empire during the early days of Christianity.
Step-by-step explanation:
The Roman engineering marvel that allowed Christian missionaries to establish new churches throughout the empire during the early days of Christianity was the Christian Basilica. Emperor Constantine financed the construction of massive churches, including the Basilica of St. Peter in Rome, which served as a model for other churches in the empire.
The Christian Basilica was an architectural style distinct from classical pagan forms. It featured a central nave with side aisles and an apse at one end. These larger and more elaborate buildings provided the necessary space and visual authority for the growing Christian congregations.