In every medieval religious building, art is conceived as a form of teaching. Everything that man should know – the history of the world, from the creation to the end of time, the dogmas of religion, the teachings of the saints, the hierarchy of virtues, the variety of knowledge, arts and crafts – was illustrated and taught through painting, stained-glass windows, sculpture, and architectural elements.

The entire cathedral was one great “biblia pauperum” – a bible of the poor – where the illiterate could learn, through their eyes, everything they needed to know about their faith. Through the multiplicity and variety of elements arranged according to an expert order and hierarchy, the most profound concepts of theology and knowledge were able to reach even the simplest minds. Moreover, for medieval man, the cathedral was always seen as the symbol of the New Jerusalem. The spiritual message that was entrusted to the Duomo and achieved with consistent continuity throughout the centuries illustrates the story of man’s salvation fulfilled by the incarnation of the Son of God through the Virgin, to whom the cathedral is ultimately dedicated.

The large central apse window, intentionally facing east with a sculpture woven into the marble structure of the rose window, attaches meaning to the entire temple.

At the center, the rays of the flaming Sun of Justice that rises in the east symbolizes Jesus Christ; to the right, the Virgin Mary receives the Annunciation from the Archangel. Close at hand, the Bishop Saints Ambrose and Galdino, bear witness to the faith of the people of Milan.

Each day, the Sun of Justice illuminates the life of man providing the ultimate dimension for his daily path, symbolized by the 52 pillars, representing the weeks of the liturgical and calendar year.

The great stained-glass windows complete the message embodied in the sculptures and narrate the story of salvation told in the Holy Scriptures: the Old Testament to the left, the New Testament to the right and, in the center, the Visions of the Apocalypse.

On his path toward God, man is accompanied in his journey on earth by the martyrs and saints positioned atop the Gothic capitals, placed along the aisles, in the presence of the people already in the Glory of Heaven. The arches supporting the dome at the great central vault crossing, symbol of Heaven made accessible to man through the crucifixion and sacrifice of Christ, feature patriarchs, prophets, kings and characters from the Old Testament who have prepared for the coming of the Savior. The four Doctors of the Latin Church are portrayed in the four cupola pendentives. Although, originally, the dome had been designed to depict an image of God seated on the throne surrounded by the four Evangelists – represented by the small spires – as described in the Book of Revelation, the cupola was later interpreted as the glorification of the Holy Mary sustained by the teachings of the Church, hence the representation of the four Doctors. Where the cupola reaches is apex, the symbolic structure is completed by the central spire atop of which stands the Assumpted Virgin Mary surrounded by four smaller spires, whose iconography is dedicated to the Madonna, amidst the rejoicing of the heavens described by the Duomo’s spires.

Inside, the theological story continues and is developed in the thick marble decorations of the vaulted ceilings of the presbytery and choir, with relief-work dedicated St. John the Baptist, the last prophet, the Virgin with Child, and the incarnation of the Word, and culminates in the large keystone of the apse semidome with the Eternal Father, beginning and end of the universe.

The portals of the two sacristies, the two earliest works of sculpture in the Duomo, summarize the symbolic message of the entire cathedral; the north sacristy exalts Christ the King and Judge; the south sacristy tells the life of the Virgin Mary.

The Duomo’s association to Mary was further emphasized by Carlo and Federico Borromeo, who with the relief-work of both the façade and tornacoro – the presbytery enclosure – wished to underline the spirituality that emerged from the Council of Trent which, in response to the positions of the Protestant Reformation, reasserted the central role of the Virgin in the salvation of the human race.