Paul Tillich, in The Protestant Era (Chicago: University of Chicago, 1948), proposed a principle which unifies all the various sects of the historical "Protestantism":
The central principle of Protestantism is the doctrine of justification by grace alone, which means that no individual and no human group can claim a divine dignity for its moral achievements, for its sacramental power, for its sanctity, or for its doctrine. If, consciously or unconsciously, they make such a claim, Protestantism requires that they be challenged by the prophetic protest, which gives God alone absoluteness and sanctity and denies every claim of human pride. . . . It implies that there cannot be a sacred system, ecclesiastical or political; that there cannot be a sacred hierarchy with absolute authority; and that there cannot be a truth in human minds which is divine truth in itself.
The Protestant Principle means that only God is absolute; everything else (scripture, church, theology, views of God) is subject to criticism because it is not God. As such, the Protestant Principle predates the Reformation and has its roots in the monotheism of Western Biblical religions. Judaism, Christianity and Islam all reject "paganism": idolatry and polytheism. There is but one God: one ultimate reality and creator of the universe. "I am the LORD thy God ... thou shalt have no other gods before me." Exodus 20:2-3. As the Prophets continually reminded the people of Israel, God is not a mere idol, demanding ritual and sacrifice: God wants his people to act with righteousness and humility, justice and compassion.
It is true that the Protestant Principle has a certain nihilistic quality inasmuch as it undermines the authority of church, dogma and tradition. For any person who considers himself a "Christian", however, at least a little rebellion, protest and nihilism is indisputably part of the Gospel message. The words and deeds of Jesus Christ allow for no other conclusion. "Do not think that I have come to bring peace to the earth;" said Jesus, "I have not come to bring peace, but a sword." Matthew 10:34. Jesus declared: "The Sabbath was made for man, not man for the Sabbath". Mark 2:27. Jesus drove the moneychangers from the Temple, declaring they had made a house of prayer into a den of thieves. Mark 11: 15 - 17; Matthew 21: 12 - 13; Luke 19: 45 - 46; John 2: 13 - 22.
Against this background, the Roman church is the historical innovation: created when Christianity became the state religion of a secular empire. The Roman Catholic Church–with its cult of the saints and purportedly infallible Church magisterium– is a syncretic blend of monotheism with the polytheistic culture and institutions of the nations which populated the Roman Empire.