The katholikon of the Hosios Loukas monastery, built around the first quarter of the eleventh century, has a narthex with a mosaic decoration on its west side. The Incredulity of Thomas is included amongst the scenes of the Passion. This paper examines the iconographic, liturgical, and dogmatic roles of the Incredulity within the context of the overall program and the liminal space of the narthex. This monumental representation evidences how depictions of the Incredulity engaged with changing beliefs concerning the senses and faith. In particular, I focus on the implications of the believer’s bi-directional movement through the narthex on entering and exiting the church. Iconographically, the representation of a door behind Christ and His wound are understood as symbolic conformation of His divine nature and a marker of the path believers should follow to attain salvation. As a confirmation of Christ’s dual nature, the Incredulity of Thomas is read in relation to funerary and Eucharistic contexts as relating to the narthex of Hosios Loukas. [Projekat Ministarstva nauke Republike Srbije, br. 177032: Tradition, innovation and identity in the Byzantine world]