The article is devoted to the unique monument of Peter the Great Baroque – the altar ciborium of the St. Petersburg Peter and Paul Cathedral. The study presents an analysis of the decoration of the ciborium (1722-1729, I.P. Zarudny). First of all, the connection of the iconographic and iconological program of the monument with the circumstances of the apostolic dedication of the cathedral of the new capital of the Russian Empire in the context of the church reform carried out by Peter I is traced. The synodal reform is proposed to be considered from the point of view of the principles of the structure of the ancient Church as a justification for the transition from patriarchal governance in favour of the conciliar Most Holy Governing Synod.In the article, the altar canopy of the Peter and Paul Cathedral is compared with the baldachin of the Assumption Cathedral of the Moscow Kremlin, with the imperial coronation baldachins of Catherine I and with the baldachin of D.L. Bernini from Saint Peter's Basilica in Rome. The external imitation of the European fashion for baldachins was perceived as an embellishing decoration, which didn't prevent the continuation of the Moscow tradition of church decoration in the St. Petersburg monument, and the preservation of its archetypal relationship with the Jerusalem temple. Thus, the semantic connection of the royal and Divine thrones, laid down in the early Byzantine period, continues until the beginning of the XVIII century. It has found its artistic interpretation, including in the composite monogram crowning the Cathedral of Peter and Paul, the variant of interpretation of which is proposed by the author. The theme of the kingdom in the decoration of I.P. Zarudny's ciborium is intertwined with the theme of apostolic succession and the foundation of the church. The apostolic dedication of the Peter and Paul Cathedral was embodied in the artistic solution of the entire altar composition, for which the image of the "Descent of the Holy Spirit on the Apostles" became central. The establishment of a new Church and a new Jerusalem, manifested in the image of "Pentecost", indicated a particularly important role for the new state symbolically denoting its continuity from both the artistic tradition of Byzantium and the tradition of the Western Christian world.