The predominant religion is southern Europe is Christianity. Christianity spread throughout Southern Europe during the Roman Empire, and Christianity was adopted as the official religion of the Roman Empire in the year 380 AD. Due to the historical break of the Christian Church into the western half based in Rome and the eastern half based in Constantinople, different branches of Christianity are prodominent in different parts of Europe. Christians in the western half of Southern Europe — e.g., Portugal, Spain, Italy — are generally Roman Catholic. Christians in the eastern half of Southern Europe — e.g., Greece, Macedonia — are generally Greek Orthodox.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): When did the Romans make Christianity their official religion?
380 AD