Early presence of Christianity in Celje is witnessed by the legend of the bishop Maximillian, a Celje compatriot, who is believed to have died a martyr's death here in 284. The new religion spread quickly in Celje and its surroundings, especially after 380 when seat of the diocese was moved to Celje as a counterpoint to the increasingly frequent Germanic invasions (Poetovia was razed by the Goths in 380). A year later, probably the first bishop of Celeia, Tenax, is referred to at the Council of Aquileia. Later references include Gaudentius and Ivan I as the last bishop of Celeia (579, 597, and 599).

Witnessing the presence of Christianity are the remains of the Old Christian basilica, the only archaeological find of its kind in the region, discovered during the construction of the Celje post office in 1897. During excavations for the foundations, the paving of an Old Christian three-nave church built in the 4th century was found. The paving, a mosaic, consisted of small pebbles of various colours. The apsis, a semi-circular elevated area for the altar and the clergy, was entirely excavated. The apsis was 13 metres wide and 8 metres long. There was a 2.7-metre mosaic strip along the wall. The pebbles were arranged into decorative shapes, mostly triangles, squares, and circles. Also interesting were the inscriptions of the names of believers and donors, especially on the central nave. The length of the preserved northern part was 22 metres, indicating the exceptionality of the Old Christian basilica of Celeia.

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