Sioni Cathedral has been the spiritual heart of Tbilisi since the 6th century. Officially the Cathedral of the Dormition of the Virgin, most locals just call it Sioni after its hilltop namesake. The current building dates mostly from the 13th century, rebuilt after the city was sacked by the Khwarezmians, though it has been repaired and reworked many times since.
Inside, the most important object is St Nino's cross, the relic of the woman who brought Christianity to Georgia in the 4th century. The cross is kept behind the iconostasis and brought out on feast days. The frescoes are 19th century, but the atmosphere is genuinely old and lived-in — this is a working cathedral, not a museum.
Sioni sits on the main street of the old town, a short walk from the Metekhi Bridge and the sulfur baths district.