It is one of the oldest churches of Leros. Its celebration is on 26 September (Repose of Saint John the Theologian).

Dating back to the 13th century, it is the most remarkable medieval monument of the island and one of the most important in the Aegean Sea.

Within the framework of the restoration of this monument, which was financed by the 3rd CSF, excavation research was carried out in Aghios Ioannis Theologos, in Lakki; it revealed the original three-nave basilica with wooden roof, founded directly on the soft rock of the area.

It had a narthex, very few remains of which have been saved. The colonnades of the naves consisted of columns and capitals of different kinds, making it hard to discern which Early Christian monuments of the area they came from.

The church has a synthronum, at the sanctuary arch, and arched windows on the surrounding walls, a triphora one on the main arch and biphora ones on the western and northern sides.

The brick walls were covered with a thin layer of coating, interrupted by uncoated intervals highlighting the bricks, on the interior and the exterior surfaces, and on the arches of the openings, this being the original decoration of the monument.

This basilica of the middle Byzantine era did not have murals. At around 1200, possibly following an earthquake, this churched was altered and instead of the wooden roof, a domed roof with groin vaults and a dome, was constructed, supported by six pillars and two columns on the main nave, and with reinforcing arches supporting almost flat roofs on the lateral naves. This change gave the church its new look, closer to the complex architectural type prevalent at the time.