Complex sentence: By the beginning of the 4th century, Christianity had become the dominant religion in the region and towns like Serdica (Sofia), Philipopolis (Plovdiv), Odessus (Varna) and Adrianople (Edirne) were significant centres of Christianity in the Roman Empire.
Split into simpler sentences: By the beginning of the 4th century, Christianity had become the dominant religion in the region. Towns such as Serdica (Sofia), Philipopolis (Plovdiv), Odessus (Varna) and Adrianople (Edirne) were significant centres of Christianity in the Roman Empire.