The basilica is the largest and most significant monument of Aquileia and is the result of a series of buildings whose roots date back to the second century A.D. In fact, this is the time of construction of the first hall of worship documented by a beautiful remnant of the polychrome mosaic floor, of Gnostic-Christian origin, whose symbolism can be connected with surprising fidelity to the Gnostic treatise of "Pistis Sophia".
Later, in the early fourth century, Bishop Teodoro modified the Gnostic hall, incorporating it in a place of worship with two parallel halls. A few years later, these floors were buried for the construction of an even wider basilica, structured with two parallel halls, a baptismal font to the west and a residential building to the north. This early Christian complex, unique in that time, became the first of many complexes known as "twin basilicas"
The devastating raids of the Hungarians and the strong earthquake of 998 heavily damaged the basilica. A major new renovation of the religious complex was therefore performed at the beginning of the eleventh century by patriarch Poppone (1019-1042), who completed the restoration of the basilica and built, with the stones of the amphitheatre, the mighty bell tower. The new complex was solemnly consecrated on July 13, 1031, and dedicated to the Virgin Mary of the Assumption and Saints Ermacora and Fortunato, as can be read in the dedicatory inscription under the great fresco of the apse, which portrays the same patriarch, the emperor Conrad, his wife Gisella and their son Henry III.
The basilica underwent another major restoration after the disastrous earthquake of 1348, by patriarch Marquardo of Randeck (1365-1381). The Romanesque arches, above the Popponian capitals, were replaced with Gothic ogival arches to support the new walls of the nave and the wooden ceiling was given the characteristic shape of "overturned ship's hull."
During the Renaissance there were other minor, non-structural interventions, but they were relevant to the theatrical aspect of the interior, such as the main altar in the apse, the marble covering the "crypt of the frescoes", the stairs, the altar of the ciborium and the "magna" tribune decorated by Benardino da Bissone.
The crypt of the frescoes, although dating back to the Massenziana age (ninth century), is best known for the frescoes with scenes of the "Passio" of the first bishop of Aquileia Ermacora and his deacon Fortunato, dating back to the twelfth century. This hall, protected by a robust gate between the columns, guarded the "religious" treasure of Aquileia, later dismembered when the Episcopal seat of Aquileia was abolished to make way for the new seats of Gorizia and Udine (1751). The gate was unfortunately taken away around 1950/60, when the entire basilica was affected by restoration works ordered by the Superintendence. The scenes of the "Passio" can be found again both around the apse (frescoes from the fourteenth century.) and in the paintings on the back of the stalls of the Aquileian Chapter's canons (fifteen century).
Excavations carried out about a century ago under the floor of the basilica of Aquileia have unearthed a large mosaic, dating to the fourth century AD, which covered a wide hall (southern hall) much more ancient than the current basilica. The mosaic carpet, of fine workmanship, is well preserved and missing only in correspondence of the majestic columns of the basilica and some graves. The excavations, subsequently performed on the outside, have unearthed the remains of another hall (northern hall), partly dating back to the same period as the southern one, which is also covered by a vast mosaic carpet, missing almost only in correspondence of the bell tower that rises above it from 1031. The mosaics of the northern hall are made of less precious materials and created by less experienced hands, except for an area, located north of the bell tower foundations, which shows older mosaics with bright colours, made with the finest workmanship
These mosaics are not classifiable in a historical/cultural Latin or Latin/Christian context. Their examination, however, provides clues and working hypotheses that move the investigation towards the Gnostic cultural circles, present during the first centuries of the Christian era all over the Roman Empire, and especially in Alexandria. In particular, the scholar Renato Iacumin sees in the older mosaics of the northern hall the iconographic illustration of various Gnostic texts, including book I and IV of the "Pistis Sophia" Code.