It takes a careful study of history of the first 500 years of the church, along with reading what the early church Father's wrote. It is basically a story of how gentiles and Jews relate to each other.
For thousands of years, before Christ, gentiles were almost always pagans, and altho many Jews joined pagans, the only ones who followed God were Jews. God taught them His principles through teaching them to do certain things. Christ had died for sins in eternal time, but not in our time so they used animal blood for forgiveness as a symbol. They didn't eat scavenger animals as a symbol of keeping clean in spirit, they were circumcised as a symbol of becoming spiritual. Saturday worship was part of this. If pagans like Ruth, wanted to become followers of God they were required to become Jews and accept all these customs. Often they didn't know why they were performing these things. This went on for thousands of years.
When Christ was crucified, there were hundreds who wanted to be Christians. You read in Acts how terrible it was for the Jews to figure out what to do about all of that. They said at least those pagans have to do enough things so they would be allowed in the synagogue so they can learn about God and Christ. Some of the Jews said absolutely not, they have to be Jews and do the entire bit. Lots of our NT epistles are Paul reporting what God had to say about that. But still, the converted pagans worshiped on Saturday mostly.
In 72 Rome destroyed the Temple in Jerusalem. The Jews rebelled, and in 132 the Romans went into Jerusalem and killed thousands and thousands of Jews. Jews were either dead or left Jerusalem, and the Jewish leaders of the church were all gone. Gentiles became leaders. Most of the Jews still living were not Christians, the Christian Jews were pretty much gone. The pagans were used to Sunday worship, not Saturday worship. When they established their own meeting places, it was usually on the Sunday they were used to. The church leaders began writing about those terrible Jews, how they all were responsible for killing Christ.
Then along came a Roman ruler (Constantine) who made Christianity legal. He hated the Jews, but Christians were OK. He opened up the world to Christianity. But he told all the pagans they weren't to worship their Gods any longer, now they were to worship Christ. He had to make this acceptable to them or he would have rebellion on his hands. He gave us the Nicene Creed, and united the Christians into one religion, establishing Christ as God. But he changed all our holidays so they would be acceptable to the pagans and so we would be rid of the Jews Constantine hated. Before, Christianity was an outgrowth of what the world had knows as the one true God, now it was a new religion. So we have Easter, Christmas, and Sunday for Sabbath, and the Jewishness has been taken out of our worship.
The history of this from 325 on is fascinating, with church councils most years, and a wonderful history of the Jews. Our churches are still reflecting Constantine more than Paul.
