Final answer:
The ritual associated with centrally planned buildings in Early Christian practice was baptism. This initiation ritual symbolized the cleansing of the spirit and marked the entry of new members into the faith.
Step-by-step explanation:
The ritual often associated with centrally planned buildings in Early Christian practice is the ritual of baptism. Early Christians started constructing larger and more elaborate buildings for worship after the end of their persecution. They adopted the basilica plan for some of their churches, with a long axis focusing on the altar. However, in the Eastern (Byzantine) Empire, centrally planned churches became common, characterized by a central dome and an ambulatory. These centrally planned churches emphasized the interior space for the mystery religion of Christianity, which required initiation - often through baptism - to participate in the rituals. Baptism was a crucial initiation practice that focused on the cleansing of the spirit, conducted by church hierarchy figures like bishops or deacons and symbolizing the entry of new members into the Christian faith.