In the center of the village, in Syros is the Orthodox church of Agios Ioannis. The church was built in 1876 with donations of Themistocles Petrokokkinos and George Veltsos, busts of which have been placed in the churchyard. Zannis Petrokokkinos sent from St. Petroupli where he lived, precious sacred objects and significant religious value icons of the 19th century that adorn the interior of Agios Ioannis.
The church is Byzantine style domed cruciform externally and painted in blue while the dome and other embellishments are painted blue. The festival of Agios Ioannis is on 29th of August and attracts many devotees from all over the island.
Especially over the past decades, people from all over the island came that day to Agios Ioannis in Posidonia and camped in the surrounding areas and the steps of the church. On the eve, in the surrounding mansions lit torches to illuminate the night, while walking the beach sailed floodlit owned vessels to participate in the celebration.
After vespers, it was the festival with live music, singing and dancing and the morning of the festival has followed the procession of the image of Agios Ioannis with the band’s entourage.