One, the churches now had an official means to decree doctrine.
The Catholic Church made dogmatic definition upon its Biblical canon at the Council of Trent of 1546, reaffirming the Canons of Florence of 1442 and North African Councils (Hippo and Carthage) of 393-419.
Further, as we see doctrines introduced at later periods into the teaching of the churches, we can conclude that those doctrines are not apostolic.
After the reign of Constantine and the first general council of the church at Nicea, two very significant events occurred.
The holy book used by the early Christians is the Jewish Bible, known to Christians now as the Old Testament ('testament' meaning in this context a covenant between God and man).
But from the middle of the 1st century AD texts begin to be written which will later be gathered into a New Testament, representing the updated covenant revealed by Christ.