History has had two main, and very different, varieties of "Protestants" - those who, for the most part nominally (i.e. in name only) came out of the Church of Rome, such as Martin Luther in the 16th century who rejected the corrupt leadership of the Pope in Luther's 95 Theses, but nevertheless generally kept most of the Pope's doctrines (hence the ease with which the modern "ecumenical" movement will see a re-uniting of those "daughter" churches under the control and authority of the Papacy, particularly when his great "miracles" begin i.e. Revelation 13:13-18), and those who were never in the Church of Rome, but instead always steadfastly maintained the teachings of the early church, as founded by Jesus Christ and the apostles i.e. truly apostolic.
The Other "Luther"
Although he is largely unknown today, Arius (250-336), a Christian teacher of Alexandria, caused a split in the Roman empire that at least equaled that of Martin Luther (1483-1546) centuries later. Among Arius' teachings were that God was absolutely One, that the Son of God was created and therefore not eternal or equal with the Father, and that the Holy Spirit was the power of the one God, not a "person" of a Trinity. He was also a seventh-day Sabbath keeper who opposed the change to Sunday, and he observed Passover rather than Easter. For those teachings, along with his widespread religious and political popularity, Arius was feared and hated (i.e. their hate was based on their fear of being unable to refute his teachings in the Word of God - they had to rely on their own church philosophers whose opinions were regarded on the same, or higher, level as the Bible) by the empire's bishops, which included the one at Rome, who at that time was just beginning to rise to "Pope."
Arius' teachings greatly influenced the three Germanic "Arian Kingdoms" - the Burgundians, the Vandals (the modern term vandalism is derived from their practice of completely devastating enemy areas that they defeated, including when they sacked Rome) and Ostrogoths - often hostile and competing, but eventually allied for a time kingdoms within the territory of the Roman empire (the Burgundians in what is today southern France, the Vandals in north Africa and the Ostrogoths in what is today Ukraine, near the Black Sea). During that same era, the successive bishops of Rome, as set up by the emperors of Rome (i.e the Emperor chose the bishop from his own city, a natural and practical political choice), were given the religious leadership over the entire area of the empire under the Roman emperor's control, including over their previously equal-with-him fellow bishops (by the very definition, bishop meant a local or regional leader - never a supreme leader), thereby becoming the sole Pope (all of the bishops were addressed as Pope before then).
The Arian Kingdoms were regarded as a serious threat to the leadership of the bishops of Rome, since they were religious adherents of Arius who would take religious control wherever they took political control (which is also what the Emperors and Popes have always done), so with the political and military power of the Roman emperors backing them, the Popes (the other "horn" that came up among them and "spoke great things" in Daniel 7:8) had the Burgundian, Vandal and the Ostrogoth kingdoms destroyed - the "three of the first horns that were plucked up by the roots" from the Roman empire (same verse). Thereafter the Church/Bishop of Rome became the "woman" riding the "beast" of the Roman empire (Revelation 17:3-18).
Fact Finder: How were the empires of Babylon, Persia, Greece and Rome symbolized in Prophecy? See Daniel's Statue