Christianity is an Abrahamic, monotheistic religion based on the life and teachings of Jesus of Nazareth.
Religious syncretism is the blending of two or more religious belief systems into an existing system, or the incorporation of beliefs from unrelated traditions into an existing religious tradition.
This can occur for many reasons, but the most common is when a culture is conquered, and the conquerors bring their religious beliefs with them, but they fail in entirely eradicating the old beliefs or, especially, the old practices, they incorporate the local culture/religion to create something familiar to ease conversion of the populace.
Harper's Bible Dictionary states that syncretism is "either a conscious combining of two or more religions over a short period of time, or a process of absorption by one religion of elements of another over a long period of time. In both types the absorbed elements are usually transformed and given new meaning by the fresh context. The borrowed item may remain outwardly the same but its new context signifies something quite new."
Attempts to assimilate other religions, or even aspects of other religions, into Christianity always meant something is either given up or added. This was the danger Paul spoke of in writing to the Colossians. The heresy within their midst was not any of the orthodox teachings, but that they were adding to it elements of Judaism and paganism (such holidays). Yet the point of the Bible is that Jesus Christ is to be worshipped as prescribed by God. Any blending with other religious ideas or practices goes against that.
Gnosticism was an early form of syncretism that challenged the beliefs of early Christians. In the first few centuries after Jesus was crucified, there were various competing "Jesus movements". The Roman emperors (starting with Constantine) used syncretism (as Rome had for centuries before) to help convert the Pagans. Then came the newly Christian theocratic empire (which in and of itself is against the bible). Social conversion to Christianity happened all over Europe for centuries (willingly or forced, the latter going against the bible). It became even more effective when missionaries encountered different cultural/religious traditions and interlaced them into a Christian synthesis to ease conquering/converting. Sometimes old pagan gods (or at least their aspects and roles) were transferred to Christian saints (St. Bridgit anyone?).
The Second Apology of Justin Martyr states: "Whatever things were rightly said among all men, are the property of us Christians." The Church has assimilated many (though not all) of the ideas of Plato and Aristotle. Augustine of Hippo is remembered for assimilating the ideas of Plato, while Thomas Aquinas is known for doing so with the ideas of Aristotle.
Romans ran on the concept of syncretism because from their earliest times they had used it with, among others, the Greeks. The Roman acceptance of other cultures' religions into their own made it easy for them to integrate the newly encountered people and existing religions they found as a result of their expansion. In other words, it was done to solidify their conquering of other cultures and religions. One can contrast Christian syncretism the practice of making Christianity relevant to culture. Some Jesuit missionaries used local religious/cultural systems and images to teach Christianity, as did the Portuguese in China.
Historian Yuval Noah Harari argues in Sapiens: A Brief History of Humankind (2014) that syncretism is pervasive in Christianity, saying that "In fact, monotheism, as it has played out in history, is a kaleidoscope of monotheist, dualist, polytheist and animist legacies, jumbling together under a single divine umbrella. The average Christian believes in the monotheist God, but also in the dualist Devil, in the polytheist saints, and in animist ghosts."
Constantine I was the first to make it legal for Christians to practice their faith openly. By the late fourth century, emperor Theodosius established Christianity as the official religion of Imperial Rome (creating a theocracy, which goes against the teachings of Jesus). Christians were then able to hold positions of power in the government (which is against the bible), which then granted the religion more followers and power. That's where it all started.
When the western Roman empire fell completely by 476, the infrastructure of the church survived, allowing more religious control in Europe. The pope became the key figure of religious leadership; bishops then controlled governments at the local level, ensuring the theocracy stayed. What followed were centuries of bloodshed.
Once the political/governing power of the church was established, the church had to keep it. Creating armies, adding rules and laws that gave the church absolute power, and using religion as a justification for the cruelty and hatred that has existed for centuries.
In "Romanism in the light of prophecy and history: its final downfall, and the triumph of the church of Christ" [New York, American and Foreign Christian Union, 1854, p. 58], Brownlee quotes a figure of 68,500,000 killed by the Papacy, composed of 50 million Christians in Europe, 15 million Indians in the New World, 1.5 million Jews in Spain and elsewhere, and 2 million Moors in Spain. He then writes, "And, O merciful Father in heaven, this does not include the millions of their own people, and her enemies, which fell in her crusades, and wars, and massacres! Here thirty million and a half would be a moderate calculation! Thus, Rome papal has hurried into eternity A HUNDRED MILLION OF THE HUMAN RACE, by her bloody religion!" That's how the church kept power.
In general, whenever the church its territory, a significant fraction or even a majority of the population was killed, as occurred in Bohemia, Spain, Central and South America, with the Waldenses, and in the extermination of the Vandals, Ostrogoths, and Heruli. Therefore the number killed was larger than the number remaining. Extending this proportion to all of Europe with a population of 50 to 100 million at the time yields an estimate of 50 to 100 million or more killed in Europe alone. For example, the conversion of Prussia to Christianity in the thirteenth century was accompanied by deliberate extermination by the Teutonic Knights.
Now, does any of that sound Christ like? And that doesn't even consider modern day.
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