This paper examines the mosaics commissioned by Pope Gregory IV (827–844) in the Basilica of S. Marco in Rome. Before Gregory IV’s reign, during Pope Paschal I’s pontificate (817–824), apse mosaics represented a space for messages about the orthodoxy of the veneration of religious images. Paschal I did not only represent the papal perspective on images in apse mosaics, but he also wrote to the Byzantine Emperor Leo V (813–820) about the validity of religious images soon after the start of the second iconoclasm (814–842). The letter, Paschal’s references to eastern theory, and the arrival of Greek iconophile refugees indicate that the controversy remained relevant in Rome even in 830, when Gregory IV’s mosaics were created. This paper argues that the ideas of Paschal and those of Byzantine iconophile theologians were considered when the mosaics of S. Marco were designed. Moreover, the enigmatic appearance of St Mark the Evangelist as the titular saint in the apse mosaic of the basilica formerly dedicated to Pope Mark is part of this iconophile message. Mark the Evangelist’s representation is deployed to claim his relics in Venice, however, his figure also emphasizes the relationship and role of words and images in the context of a common iconophile argument about the Incarnation of Christ. This process of associations is aided by the composition of the mosaic itself. The mosaic emphasizes the connection between the saints of the apse, which allows them to indicate messages about the primacy of the bishop of Rome and his orthodoxy in the defence of images.