|The old cathedral|
The first reference to the Old Cathedral was made in a golden bull of Adronikos the Senior in 1321 AD. It is described in detail by the orator Theodore Pediasimos, a native of Serres, in his brilliant essay "expressions about the sanctum of Feres (i.e. Serres)" (14th century). At the northwest side there is a burial chapel, probably built in 1364 for Patriarch Kallistos. A plinth sign on the southern side of the church wall states the following: "ΕΝ ΕΤΕΙ ΕΨΚΕ + Ο ΤΑΠΕΙΝΟΣ ΜΗΤΡΟΠΟΛΙΤΗΣ ΣΕΡΡΩΝ ΣΤΕΦΑΝΟΣ Ο ΚΤΙΤΩΡ", testifying that the wall was demolished and rebuilt in 1725.
St. Theodores' church is well-known for its mosaics, from which after the fire of 1913 the only one that was rescued was the one that shows Apostle Andreas and the hands of Apostle Loukas. In the apse is a huge mosaic of the Last Supper, still splendid in its decay. This historical church has been repaired and renovated during the years, which has resulted in a slight transformation of its original shape. During the great fire of 1913 the church got almost completely burned. It was restored recently under the watchful eye of the Archaeological Authorities and church services are now held there.