When and how did Christianity become legal?

Answer 1

The Roman Emperor Constantine...

The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia (present-day İznik in Turkey) by the Roman Emperor Constantine I in A.D. 325. The Council was historically significant as the first effort to attain consensus in the church through an assembly representing all of Christendom.

Its main accomplishments were discussion and settlement of the Christological issue of the relationship of Jesus to God the Father; the construction of the first part of the Nicene Creed; settling the calculation of the date of Easter; and promulgation of early canon law

Answer 2

AnswerIt is hard to say that Christianity was ever illegal, in order to become legal. What was illegal in the Roman Empire was Atheism, and Christians were sometimes accused of being atheists, because they refused to worship the Roman gods.

When the pagan Roman Emperor Severus Alexander (222-235 CE), placed the statues of Abraham and of Christ in his domestic chapel as a mark of respect for two minority religions, he may have been declaring that Christianity should be considered legal if this was not already the case..

Answer 3

When Constantine declared he had a vision indicative of Christianity and was a christian and as thus declared it legal.