The mosaics of the Basilica were discovered during the first archaeological campaign of 1933. They were studied and published by Fathers Saller and Bagatti . This phase can be divided, typologically and chronologically, into two phases, except for the mosaic floor of the cella trichora, whose decorative motif was partly reduced in the northern and southern semicircle by the inclusion of the two separating walls of the sacristies.
a) The mosaics of the basilica proper to which can be added the intermediate mosaic of the narthex in the fašade.
b) the mosaics of the southern area (new diakonikon) and the two northern chapels (the new baptistery and the Theotokos chapel)
The archaeological analysis, the dates and the names of the bishops and abbots remembered in the inscriptions, give us the possibility to chronologically distinguish the mosaics of the two northern chapels which were added in two distinct periods. The mosaic of the baptistery was completed at the end of the VI century, in 570. In the first or second decennial of the VII century the addition of the Theotokos chapel was brought to an end.
The date of the construction of the basilica and its mosaic floor remains uncertain.
The mosaic of the basilica proper
The decorative plan of the basilica was imagined as being a unified composition enclosed in a double band composed by a T-shaped meander of simple swastikas of alternate red and black tesserae . The meander was surrounded by a ribbon made up of vine shoot with tendrils and ivy leaves that surrounded the whole perimeter of the church. The vine shoot changes form on the western end of the two lateral naves. Here it covers a larger area between the meander which follows the step of the presbytery and the walls. Two flower crosses break the vine shoot at the northern semi-pillar level towards the fašade. On the eastern end of the northern nave, in the space between the vine shoot and the step of the presbytery two diamonds with flowers at the corners are added.
Judging from the few surviving remains in the vicinity of the columns towards the centre of the basilica, the central nave was decorated with a composition of scuta crosses having two-stranded guilloche alternating with cordoli rendered using the rainbow technique. On the sides, the composition extended up to the alignment of the columns. It was broken off on the eastern end by a series of animals adjacent to vegetable motifs, alternated with small trees. The base of the pulpit of the basilica, which was added in a second instance at the south eastern corner of the central nave, saved from destruction part of the figurative motif.
The isolated small tree is summarily executed with the superimposition of chromatic plains. A line of black tesserae closes the schematic outline having a grey point, of the first plain of foliage on the outside. A double line of red tesserae is added internally to outline, in the same fashion, a continuous plain of yellow tesserae, interrupted at the centre by a schematic representation of a small tree, which is developed in height, using red tesserae. From the trunk, there departs a stumpy branch rendered with yellow tesserae and outlined with a line of black tesserae. The animal, of which remains only the hind part, is also rendered with the same summary technique. A black contour line follows, to highlight the form of the figure rendered with yellow tesserae. The animal's fur is characterized with cuneiform tufts of red tesserae. A line of red tesserae underlines the animal's testicles, while the split hoofs are rendered with triangles of black tesserae. Lines of black, yellow and red tesserae are utilized to render the vegetable motif, issuing from a cone-shaped trunk and having small side branches, which rises behind the animal
The majority of the inter columnar panels survived. Their decorations are all taken from the geometric repertoire: a network of flowers with the resultant rhombi containing diamonds, a checkered motif, intertwined circles to form a trifoliate design, knotted circles containing diamonds, series of circles alternated with diagonally placed squares, knotted circles decorated with lozenges.
The mosaic of the lateral naves survived almost in their entirety. In both cases the decorative plan was divided in three distinct panels.
In the northern nave a network of flowers having the resultant rhombi laden with opened corollae, is followed in the central panel by an interlacement of circles laden with diamonds, alternated and knotted to squares containing knotted squares. The northern panel of the composition is made up of a continuous superimposed series of polylobed octagons with a grid of lunes made of black tesserae with the addition of flowers.
In the southern nave the western panel, made up of knotted rhombi, and the subsequent panel wherein the motif with interlaced circles alternated with squares is repeated, are divided by a dedicatory inscription with the name of the benefactor Anosa and his family. A grid of rhombi containing flowers closes the composition towards the east.
Lacking other information the most similar parallel for dating this work can be found in the mosaic of the northern church of Hesban and that of the church of the Holy Fathers at Khattabiyah.
In the mosaic of the church of Hesban, dated towards the end of the V century - beginning of the VI, the trees are either treated as one body with overlaying layers of colours, red and yellow, red and black like the small tree to the right of the animal in the mosaic of Nebo, or else like the branches with curved leaves as the plant next to the animal . The animals are rendered with the same approximate technique as the surviving beast at Nebo . Furthermore, even the building, of which the above mentioned mosaic forms an integral part, bears very close resemblance to the basilica at Nebo: the capitals with the foliage decorations seem to come from the same workshop; the name of the benefactor Philadelphio might have some connection with the scholar benefactor of the diakonikon-baptistery; the letters of the dedicatory inscription in front of the step of the presbytery are very similar to those of the dedicatory inscription with the name of the benefactor Anosa in the southern nave of the basilica of Moses.
Unfortunately dating the church of Khattabiyah is difficult given that no dates are available and there is also a lack of archaeological documentation. Here too we find the decorative pattern of the basilica of Nebo with the double meander borders and the ribbon formed by the vine shoot with tendrils and ivy leaves which runs along the internal perimeter of the church and the motif a croci di scuta of the central nave .
Keeping in mind the archaeological evidence and the analysis of the architectural complex in its totality we can certainly date the northern church of Hesban and its mosaic to the first decades of the VI century. The construction of the basilica of Moses has to be placed after the decoration of the diakonikon-baptistery (530), in the first half of the VI century, in a distinct phase from the construction of the chapel of the new baptistery (597).
In this hypothesis we have to admit that the diakonikon-baptistery remained in use only for some decennials before being covered up and substituted by the new diakonikon.