In the Roman Empire, a basilica was a public building for justice, trade and administration. This type of building had an elongated shape with a nave and aisles, often with an apse (semicircular niche) at the end where the right one sat. When Christianity emerged, these basilicas were copied as a template for churches, because they were large, stately and suitable for gatherings.
In 326 – 360, Emperor Constantine had St. Peter’s built. That was the first Christian basilica, inspired by the Roman courthouses in terms of shape. But always intended as a Church. The basilica was built on the Vatican Hill: the place where Pertus is buried. (He also had a large basilica built for Saint Paul on the road to Ostia)
The shape of such a basilica symbolized “order” and “authority”. Originally at the emperor’s courthouse, but at the Church it was the order and authority of Christ.