Before it named a church, basilica named a Roman civic building, a long rectangular hall used for law courts, markets and public administration, entered along its length and often ending in a semicircular apse where a magistrate sat to hear cases. When Constantine legalized and then patronized Christianity in the early fourth century, this secular hall, rather than any earlier temple form, became the model for large scale Christian building, because its open interior suited congregational assembly in a way a Greco-Roman temple, designed to house a cult statue rather than a crowd, did not.
Constantine's own building program produced the first great Christian basilicas in Rome, including Old St Peter's and San Giovanni in Laterano, both raised directly on the civic basilica plan: a long nave flanked by lower side aisles, a row of columns marking the division, and the apse, once the magistrate's seat, now holding the altar. This plan proved durable precisely because it was structurally simple, requiring no advanced vaulting technology, and it remained the basic template for church building across the Mediterranean world for centuries, long after more elaborate domed and Gothic forms developed alongside it in particular regions and periods.