The highlight of this monument is the spectacular 3rd- to 4th-century mosaic covering the entire floor of the basilica and the adjacent crypt, comprising one of the most beautiful and important of early-Christian monuments. Theodore, the basilica's first bishop, built two parallel basilicas, now the north and the south halls, on the site of a Gnostic chapel in the 4th century. These were joined by a third hall, forming a U, with the baptismal font in the middle. The complex was rebuilt between 1021 and 1031, and later accumulated the Romanesque portico and the Gothic bell tower, producing the church you see today. The mosaic floor of the present-day basilica is essentially the remains of the floor of Theodore's south hall, while those of the Cripta degli Scavi are those of his north hall, along with the remains of the mosaic floor of a pre-Christian Roman house and warehouse.
The mosaics of the basilica are important not only because of their beauty, but also because they provide
a window into Gnostic symbolism and the conflict between Gnosticism and the early-Christian church. In his north hall, Theodore retained much of the floor of the earlier Gnostic chapel, whose mosaics, done largely in the 3rd century, represent the ascent of the soul, through the realm of the planets and constellations, to God, who is represented as a ram. (The ram, at the head of the zodiac, is the Gnostic generative force.) Libra is not the scales, but rather a battle between good (the rooster) and evil (the tortoise); the constellation Cancer is represented as a shrimp on a tree. The basis for the representation in Aquileia is the Pistis Sophia, a 2nd-century Gnostic tract written in Alexandria.
This integration of Gnosticism into a Christian church is particularly interesting, since Gnosticism had already been branded a heresy by influential early Church fathers. In retaining these mosaics, Theodore may have been publicly expressing a leaning toward Gnosticism. Alternatively, the area of the north hall may have been Theodore's private residence, where the retention of Gnostic symbolism may have been more acceptable.
The 4th-century mosaics of the south hall (the present-day nave of the basilica) are somewhat more doctrinally conventional, and represent the story of Jonah as prefiguring the salvation offered by the Church.
Down a flight of steps, the Cripta degli Affreschi contains beautiful 12th-century frescoes, among them Saint Peter sending Saint Mark to Aquileia and the beheading of Saints Hermagoras and Fortunatus, to whom the basilica is dedicated.