On God’s Chariot
While the choir sings, the bishop goes forth as if driven on a chariot, for it is written that the chariot of God  is attended by ten thousands, and the bishop’s retinue is divided into ten orders, namely: the first order are the porters; the second, the lectors; the third, the exorcists; the fourth, acolytes; the fifth, the subdeacons; the sixth, the deacons; the seventh, the presbyters; the eighth, the cantors; the ninth, the laymen; the tenth, the women. Thus Christ came into the world, while the choir of prophets sang, driven on the chariot of Scripture, accompanied by the orders of saints.
The cantors welcome the bishop as he enters with the Gloria Patri, and the angels received Christ as he was born with the Gloria in excelsis. The two choirs sing praises together, because two peoples—to wit, the Jews and the Gentiles—ran to meet Christ with praises.
In the Introit chant, the Church’s praise is taken up by the Jews; in the Kyrie eleyson withal, the Church’s praise is taken up by the Gentiles. In the Gloria in excelsis, however, both sing praise together faithful to the Trinity in order to secure a dignity equal to that of the angels.
In the Introit, too, we are shown the order of patriarchs, who foretold the coming of Christ. The prophetic verse signifies the order of prophets, who signaled the birth of Christ. The Gloria Patri commemorates the order of apostles, who preach that Christ has already come, and who expound the Trinity to the Church. The repetition of the Introit alludes to the order of doctors, who announce that Christ will come again as a judge.
Further, the Kyrie eleyson proclaims the peoples of diverse tongues by whom Christ is praised, together with the angels, in the Gloria in excelsis.
 Louis Marshall explains that “certain early Christian writers used the metaphor of the antique quadriga, or two-wheeled chariot drawn by a team of four horses, to evoke the character and action of the four Evangelists. Under the guidance of Christ as their charioteer, the four authors of the Gospels act as a team, vigorously drawing forth the word of God into the world” [Parergon 13.2 (1996): 260-262].
Some have argued that the four horses placed above the portal of St. Mark’s cathedral recall this ancient tradition (See Jacoff, The Horses of San Marco, pp. 21-26). Under this interpretation, looking closely at Bellini’s Procession painting, the whole massive structure of St. Mark’s can be viewed as a Quadriga Domini:
De curru Dei.
Interim dum chorus cantat, episcopus quasi in curru vectus ad solemnitatem vadit, quia currus Dei decem millibus multiplex legitur, et comitatus episcopi decem ordinibus distinguitur, scilicet: Primus ordo, sunt ostiarii; secundus, lectores; tertius, exorcistae; quartus, acolythi; quintus, subdiaconi; sextus, diaconi; septimus, presbyteri; octavus, cantores; nonus, laici; decimus, feminae. Ita Christus mundum intravit, dum prophetarum chorus cecinerunt, curru Scripturae vectus, sanctorum ordinibus comitatus. Cantores venientem episcopum cum Gloria Patri excipiunt, et angeli Christum advenientem cum Gloria in excelsis susceperunt. Duo chori laudes concinunt, quia duo populi scilicet Iudaei et gentiles Christo advenienti cum laudibus occurrerunt. Per cantum Introitus, accipitur laus Ecclesiae de Iudaeis; per Kyrie eleyson vero laus Ecclesiae de gentibus. Per Gloria in excelsis autem utriusque concors laudatio in fide Trinitatis pro adipiscenda aequalitate angelicae dignitatis. Per introitum quoque ordo patriarcharum nobis repraesentatur, per quos Christus venturus praefigurabatur. Per versum propheticum ordo prophetarum insinuatur, per quos Christus nasciturus pronuntiabatur. Per Gloria Patri ordo apostolicus commemoratur, per quos Christus iam venisse praedicatur, a quibus et Trinitas Ecclesiae insinuatur. Per introitum secundo repetitum ordo doctorum notatur per quos Christus adhuc venturus ad iudicium narratur. Porro per Kyrie eleyson diversarum linguarum populi declarantur, a quibus Christus in Gloria in excelsis cum angelis collaudatur.