A couple of good historians that talk about the corruption of the Christian congregation (church) after Jesus died are Edward Gibbon and Will Durant. I think Edward Gibbon did a good job in his very wordy
The History of the Decline and Fall of the Roman Empire. It's been years since I read either.
Even Jesus foretold an apostasy in the congregation after he died. He explained about it in an illustration of a sower of seed and a field. I hope you don't mind my going into some detail in answering your questions above. As I have some time and my mind is on it I thought I would.
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He presented another illustration to them, saying: “The Kingdom of the heavens may be likened to a man who sowed fine seed in his field. While men were sleeping, his enemy came and oversowed weeds in among the wheat and left. When the stalk sprouted and produced fruit, then the weeds also appeared. So the slaves of the master of the house came and said to him, ‘Master, did you not sow fine seed in your field? How, then, does it have weeds?’ He said to them, ‘An enemy, a man, did this.’ The slaves said to him, ‘Do you want us, then, to go out and collect them?’ He said, ‘No, for fear that while collecting the weeds, you uproot the wheat with them. Let both grow together until the harvest, and in the harvest season, I will tell the reapers: First collect the weeds and bind them in bundles to burn them up; then gather the wheat into my storehouse.’”-Matthew 13:24-30.
The disciples did not understand this illustration and after the people left Jesus they asked him what it meant.
Jesus said that he spoke by means of illustrations so that those who did not have the right heart condition would not understand it. They did not appreciate spiritual matters and did not take the time to stop and investigate them further.
The disciples did have the right attitude and asked for understanding and it was given to them:
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His disciples came to him and said: “Explain to us the illustration of the weeds in the field.” In response he said: “The sower of the fine seed is the Son of man; the field is the world. As for the fine seed, these are the sons of the Kingdom, but the weeds are the sons of the wicked one, and the enemy who sowed them is the Devil. The harvest is a conclusion of a system of things, and the reapers are angels. Therefore, just as the weeds are collected and burned with fire, so it will be in the conclusion of the system of things. The Son of man will send his angels, and they will collect out from his Kingdom all things that cause stumbling and people who practice lawlessness, and they will pitch them into the fiery furnace. There is where their weeping and the gnashing of their teeth will be. At that time the righteous ones will shine as brightly as the sun in the Kingdom of their Father. Let the one who has ears listen."-Matthew 13:36-43.
See:
The Great Apostasy Develops — Watchtower ONLINE LIBRARY a good explanation of the apostasy that developed in the Christian Church after Jesus' death.
Here is the chapter in Edward Gibbon's voluminous work that goes into some detail about this falling away and apostasy of the truth:
https://erenow.net/ancient/fallromeempire/volumei/26.php
The idea that Jesus was god was not taught by his original apostles or followers. But in the succeeding centuries after his death many sects of Christianity developed, which eventually lead to the Athanasian Creed.
The Protestant Reformation was a protest against many of the abuses of the Catholic Church and the schism was further driven among the warring sects of Christendom. But the reformations did not get rid of many of the pagan teachings that had seeped into the Church and corrupted Christianity over the centuries. It was not God's time.
Notice that it would be during the time of the end that the wheat would be separated from the weeds. The weeds appear as wheat, they are imitation Christians.
Jehovah's Witnesses are not a sect of Christendom as are the many branches of Protestants.
Rather they are a return to the original form, or true Christianity. This is a work of Jehovah God and not that of any man.
Remember the illustration of the wheat and the weeds.
The Sower: Jesus Christ.
He sowed the fine seed while on earth and preached about the good news of God's kingdom gathering followers of the truth.
The field: The world of mankind.
Jesus sowed the kingdom truth in people's hearts as he preached about God's kingdom. Some people listened to the message.
The fine seed: The sons of the kingdom. True anointed Christians.
The Weeds: The sons of the wicked one.
The Enemy: Satan the Devil.
Satan sows fake, or imitation Christians into the Christian congregation. In the beginning true Christians began to apostatize, that is turn away from the truth and taught falsehoods to gather people to themselves. The apostates were not weeds.
Later people such as Emperor Constantine who were pagan feigned to be followers of Christ, and while he officiated at the Pontifix Maximus of the pagan religion of the Romans he claimed to be a follower of Christ. Many others followed his example. The pagan Emperor himself is the one who officiated over the Council of Nicaea and was the one who guided the discussions. This pagan was the one who got the council to agree to the concept that Jesus was God. The Trinity that added the holy spirit into the mix did not come for some centuries.
Satan had completely solidified the apostasy of the Christian Church. Now weeds were in full bloom. These were people who claimed to be Christian but never were to begin with.
The harvest: A conclusion of system of things.
Jesus in heaven would begin to separate the wheat, true anointed Christians, from the weeds, imitation Christians, in name only but who follow pagan teachings and God-dishonoring doctrines during the last days, or the conclusion of this system.
The reapers: Angels.
The angels are involved in the great preaching and teaching work of the good news during the last days. In Revelation we are told it is an angel flying in mid-heaven declaring:
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And I saw another angel flying in midheaven, and he had everlasting good news to declare to those who dwell on the earth, to every nation and tribe and tongue and people. He was saying in a loud voice: “Fear God and give him glory, because the hour of judgment by him has arrived."-Revelation 14:6, 7.
The return to true worship by Jehovah's Witnesses was foretold by Jesus Christ.
The many sects and divisions of Christendom are the weeds. Imitation Christians.
The preaching work is a modern-day miracle.
There is only one true faith and one true baptism.
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One body there is, and one spirit, just as you were called to the one hope of your calling; one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all."-Ephesians 4:4-6.
Even Jehovah himself proclaimed that he would have witnesses for his name:
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You are my witnesses,” declares Jehovah, “Yes, my servant whom I have chosen, So that you may know and have faith in me And understand that I am the same One. Before me no God was formed, And after me there has been none."-Isaiah 43:10.