Following the Edict of Milan in 313 Constantine began an extensive building program to provide churches and meeting places for Christians. Previously they met in private homes that had rooms for worship.
The first Christian churches used Roman structural and design elements.
The basilica evolved into the essential design for the church that is still used today.

Old St. Peter's
Begun 333, reconstruction drawing

Floor Plan of Old St. Peter's

Old St. Peter’s x-section

Interior of Santa Sabina, another Early Christian, timber roofed, basilica. Rome, 5th century. This may give an idea of what the interior of St. Peter's might have looked like.

Old St. Peter's (so called to distinguish it from the current St. Peters) is built over the site of St. Peter's burial
- It was as large as a Gothic cathedral but it had a timber roof (not vaulted)
- had a courtyard/atrium with a propylaeum entrance
- transept and crossing

Central Plan Churches

Santa Costanza interior 337- 351 Rome

Santa Costanza plan

Santa Costanza was built as a Mausoleum for Constantine’s daughter, Constantina.
Central plan churches such as Santa Costanza evolved from influences fr. the eastern empire including Greek tholos tombs and other sources such as the Pantheon.

Information provided by: http://www.coco.cc.az.us