• There are numerous different Proestant groups, such as Baptists or Seventh-day Adventists and each will have major differences between them.
• Emperor Constantine made the persecution of Christians illegal in the Edict of Milan of 313 A.D..
• Reformers such as Martin Luther rejected teachings of Rome, such as indulgences (Ninety-five Theses in Wittenberg Germany, 1517), where people paid the Roman Catholic Church money for their relatives to be brought out of purgatory (a half-way location where people suffered btween earth and reaching heaven) faster.
• The Roman Catholic Church asserts the primacy of the pope, whilst traditionally Protestants did not accept that.
• The Roman Catholic Church also talks itself as the "mother church", encouraging other churches to return to it, which is being seen around the world happening increasingly.

Reference: www.britannica.com/biography/Constantine-I-Roman-emperor.