While parts (1), (2), and (3) are accurate, part (4) needs revision.
The Roman Emperor Constantine did in fact legalized the Christianity, and he did that with the Edict of Milan. After issuing the Edict of Milan, he himself embraced the Christian faith and became the first Roman Emperor to do so. With this edict, the Christians were protected from prosecution, and they started to thrive and steadily grow in number.
Th number (4) statement needs revision because there was no need for Catholic leaders to gather and implement the Edict of Milan, but it was all done since its issuing.