One of the oldest and most important Byzantine ecclesiastical monuments of the region, the church of Agios Ioannis Prodromos (St. John the Baptist), is located on a hill that dominates the left bank of the Acheloos, about 1 km south of the village of Mastrou in the Municipality of Messolonghi. It belongs to the monuments of the Paracheloitis and is dedicated to Agios Ioannis, commonly known as the Riganas, whose festival is celebrated on 24 June, when the oregano (rigani) is in blossom. It is also called as Episkopi (Episcopal), as it is believed that it was probably the seat of the Bishopric of Acheloos, which flourished in the late Middle Ages.
It was originally built in the mid-7th century and was used until the mid-9th century, undergoing reconstructions in between. At its base it preserves stone material from ancient buildings that probably came from the ancient city of Paianion, located north of the village of Mastrou.
In the Early Byzantine period it stood as a three-aisled wooden-roofed basilica (16.90 x 13.60 m), with a raised middle nave, three semicircular apses and a narthex. The central semicircular apse had a stepped arrangement, an indication of artistic contacts with Macedonia and especially with Thessaloniki. It was reinforced with two buttresses built out radially. The aisles were separated by continuous walls interrupted by arched openings for their intercommunication. The entrance to the narthex was on the north side and communicated with the central nave through a triple opening and with the side aisles through arched openings. The floor of the central nave had a mosaic with geometric and floral motifs now preserved in fragments, while the floors of the side aisles and the narthex were made of ceramic tiles.
Its mural decoration dates back to the 13th century and best survives in the niche of the Sanctuary; there is a preserved representation of the Virgin Mary in the Platytera pose between two assisting archangels wearing imperial robes and holding a globe, a work that represents the artistic trends favoured by the aristocracy. In the quadrant of the niche, below Virgin Mary, there is a representation of the founder, while on the sides of the triple window are eight hierarchs in a poor state of preservation, and in the south-west corner evangelical scenes and saints in frontal poses.
In the mid 20th century the church was reduced in size to the central nave. After the collapse of the side aisles and narthex, it received alterations and additions in its fixing up.
In 1969, the Ministry of Culture (8th Ephorate of Byzantine Antiquities) carried out works of conservation and repair, under the direction of Panagiotis Vokotopoulos. More recently, works were carried out between 2011-2012 (by the 22nd EBA), in the framework of the NSRF “Western Greece – Peloponnese – Ionian Islands 2007-2013” (Project “Repair and promotion of the Byzantine church of Timios Prodromos of Mastro, Aitoloakarnania, budget 200,000 €). At the same time, excavation research was carried out and the necessary interventions were made for the ehcancement of the church to a monument that can be visited.
