This, the most important early Byzantine monument of Santorini, is located in the southern port of Kastro in ancient Thera, and is known from the literary sources as Eleusis.
It is the largest church on the island and the second largest in the archipelago after Katapoliani on Paros. It dominated the bustling harbour-town for centuries, and eventually gave its name to the island that was changed from Thera to Santa Irini, i.e. Santorini.
Initially, probably in the 6th century AD, it was erected as a three-aisled wooden-roofed basilica with balconies. In a second phase, it was converted into a domed, cruciform church, when pillars were added to the colonnades to carry the complex cruciform vaulting, at the centre of which sprang the dome. The northern extension of the complex also acquired a corresponding condensed domed arrangement.
The domed churches with a dominant cruciform arrangement date back to the 8th century and are associated with the iconoclastic tendency to express in its religious architecture the leading iconoclastic symbol of the cross. Many Early Byzantine basilicas, usually after some destruction, were adapted to the new trend. Aghia Irini in Constantinople is the most prominent example. It seems that the volcanic eruption of 726 ruined the Theran church and that the Emperor Leo III hastened to renovate it, transforming it into a cruciform domed structure. The numismatic evidence from the site confirms the dating.
The third phase of the monument is the two-aisled, vaulted mid-Byzantine church that was erected in its central aisle after the ruination that befell the domed basilica; it retains most of its roof to this day.
