From a few literary sources we know that the Emperor Constantine donated a church to Ostia. This is described at length in the Liber Pontificalis (Silvester, 314-335 AD)), where donations by a certain Gallicanus are also recorded. The church was dedicated to Peter, Paul and John the Baptist. The dedication to the latter and the donation of a baptismal installation indicate, that it was the church of the Bishop of Ostia, who from 336 AD consecrated the Pope. The same church is mentioned in the Acts of Saint Gallicanus (Acta Sanctorum, June, VII), where Gallicanus is the sole builder. Flavius Gallicanus was consul in 330 AD, and then established himself in Ostia. In the Acts the church is said to be near a gate called Laurentia, that is, near a city gate leading to Laurentum, to the south of Ostia.
A geophysical survey (combined use of aerial photographs, magnetometry, and electrical resistance tests) of the unexcavated areas of Ostia led to the discovery of this church. The survey, under the direction of the German Archaeological Institute in Rome (Michael Heinzelmann and Franz Alto Bauer), was begun in 1996. Several small trenches were investigated in 1998 and 1999. The building is situated to the north-east of a secondary gate at the south end of Via del Sabazeo.
Region V with new discoveries, resulting from the geophysical research by the German Archaeological
Institute in Rome.The Basilica of Constantine is indicated in blue. It was erected on top of a building
from the Hadrianic period, indicated in red. Numbers indicate trenches. The secondary gate at the south
end of Via del Sabazeo is at nr. 11. Plan: Michael Heinzelmann.
In front of the church is a courtyard, the atrium. Between Via del Sabazeo and the atrium was a small square, from which the atrium was reached. The actual church has three aisles, the central one with an apse. The length of the central aisle plus the apse is 51.45, the width of the three aisles is 23.20. Very little remains of the walls. There were fourteen columns on each side of the central aisle. The building must have been more than 14 metres high.
Plan of the Basilica of Constantine (blue) and older buildings (grey). Numbers indicate trenches.
Plan: Michael Heinzelmann.
The Basilica was erected on top of a Hadrianic building. The atrium reuses its courtyard. The Hadrianic building in its turn was built on top of a building from the Augustan period.
The pottery found in the trenches confirms the Constantinian dating of the church. In the later fourth or early fifth century the floors were raised. A rectangular chapel for baptism with an apse at the east end, set against the south wall of the atrium, is also post-Constantinian. Some early Mediaeval sherds were found inside the church, and in this period the apse at the end of the central aisle may have been rebuilt. In the late fifth to seventh century domestic structures were built near the church, that was slowly being abandoned. In the eighth century the building was systematically plundered and in the late eighth and early ninth century the walls collapsed, shortly before Gregoriopolis (modern Ostia Antica) was built.