Final answer:
Christians developed churches and baptisteries as they created more complex worship rites, transitioning from house churches to grand basilicas and special structures for baptism in the post-persecution era.
Step-by-step explanation:
As their rites became more ritualized and complicated, Christians developed special buildings for religious purposes. The correct completion for the sentence is churches and baptisteries. These buildings evolved as Christianity transformed from a persecuted religion to one supported by the state.
Early Christians initially met in house churches, but with the end of persecution and the backing of rulers like Constantine, more elaborate religious structures were erected, amongst them basilicas, which became a central feature of Christian architecture. Baptisteries were another specialized type of building that emerged for the initiation rite of baptism, which had great significance in the Christian faith. The basilica served the communal needs of the congregations, facilitating gatherings for worship, which played an essential role in establishing a communal Christian identity.