St Edwards Church dates from 15c, and the oldest cathedral in England is said to be Durham Cathedral, built in 1093. The Basilica of Saint Sophia, makes them both look like impudent school children with the current building dating to the 4th-6th century. The current three-nave building was itself built upon the bases of churches dating even further back, so it really does have a very good pedigree. The Basilica St Sophia is dedicated to the Holy Wisdom (Hagia Sophia in the Greek language).
In 311 the Edict of Toleration was issued in Serdica (the old name for Sofia) by the Roman emperor Galerius officially ending the Diocletian Persecution. The Edict implicitly granted Christianity the status of “religio licita”, a worship recognized and accepted by the Roman Empire. It was the first edict legalising Christianity, and preceded the Edict of Milan (313). For the next 140 years the city expanded. Constantine the Great called it “my Rome”. The Council of Sardica (343) was held in the city, in a church located where the current 6th century St Sophia Church was later built.
It is only the second oldest church is Sofia, as it is predated by the St George Rotunda which was also built by the romans in the 4th century.