What event led to the recognition of Christianity ad the official religion of the roman empire?

Answer 1

The conversion to Christianity of Flavius Valerius Constantinus as he entered a battle to invade Rome and become the Roman Emperor Constantine. As emperor he then made Christianity the state religion of the Roman Empire.

Answer 2

The co-emperors Theodosius I (or the Great) and Gratian issued Edict of Thessalonica in 380. This made 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 were branded as heretic. Theodosius started to persecute them soon afterwards. His main target was Arian Christianity, which was popular around the empire. He expelled Demophilus of Constantinople, the most important Arian Bishop.

At that time the Latin/Western church and the Greek/Eastern Church, which were the main church of the western and eastern part of the empire respectively, were two branches of one church which was called Catholic Church and they both subscribed to the Nicene Creed, a particular interpretation of the trinity. Later these two churches spit and came to be called Catholic and Orthodox respectively.

Theodosius I also persecuted paganism. He reiterated an existing ban on the customs of Roman religion, made divination punishable by death, pioneered the criminalization of officials who did not enforce anti-Roman religion laws and destroyed Roman temples. He issued further decrees (the Theodosian decrees) which removed non-mainstream Christians from church office, and eliminated the remnants of Roman religion by tuning its holidays into working days, banning sacrifices, making divination (and witchcraft) punishable, and disbanding the very important priesthood of the Vestal Virgins. His final law forbad all public customs of Roman religion.

Theodosius I also persecuted the Manicheans.