What emperor made Christianity official?

Answer 1

Co-emperors Theodosius I and Gratian issued the Edict of Thessalonica in 380. This made the Nicene Creed (a particular interpretation of the trinity) which was endorsed by mainstream Christianity (Latin or Western Christianity and Greek or Eastern Christianity) the sole legitimate religion of the Roman Empire. The purpose of the edict was to ban dissident Christian doctrines which disputed the Nicene Creed and had alternative interpretations of the trinity. These were branded as heretic. Theodosius I started persecuting them soon afterwards. The main target was Arian Christianity, which the main dissident Christian doctrine and was popular around the empire.

The Latin/Western Church and the Greek/Eastern Church were the main church of the western and eastern parts of the empire respectively. They deifned themselves as two branches of one church which was called Catholic Church. The word catholic means all embracing, universal. In other words, these two churches considered themselves as two branches of the Nicene Creed, which they wanted to be the universal creed of Chisitanity , in opposition to the dissident Chisitan creeds. Later there was a schism between these two churches and they came to be called Chatolic Chush and Orthodox Curch.