The most important reason why Christians made Sunday the sabbath was because 'Sunday' or 'the day of the sun' was already very popular in the Roman empire for the worship of the sun and Mithras.
Instead of competing with pagans on this, the Christians incorporated Sunday into Christianity by making sunday the day for worshipping the sOn rather than the sUn. Sunday was chosen for the sabbath as it made for a more smooth transition to the new religion for most inhabitants of the empire.
The Sabbath observance was eventually enshrined in imperial law.
In the same way, The Roman festival of Saturnalia was incorporated into Christianity as Christmas.
This was a fantastic strategy adopted by the Christian fathers and supported by Constantine and later Christian emperors: Instead of competing with the most popular pagan festivals and observances, which they knew they didn't have a hope in hell of banning, they instead decide to incorporate them into Christianity itself.
This allowed for a relatively smooth transition from a pagan empire to a Christian empire. The rest as they say, is history.
Yes, very true.
One of the things that has kept Christianity alive is its ability to absorb and assimilate the various religious influences it meets. It probably started with Paul (and unknown others) taking Jewish thought forms into the larger Hellenistic culture of the Roman Empire. Those of us with Western Celtic roots know how Christianity absorbed the ancient Druidic culture -- and other local Irish, Scot, Welsh and Brit traditions. How many of the Celtic "saints" are transformed Druid "saints"? My poor old grandmother would turn over in her grave -- but I dare say Brigit is one!
When I moved from the UK to the US, I married a gal who is Cherokee Indian and began to see firsthand how various Native American spiritual traditions have been "reborn" into a Christian framework. Some would be considered "orthodox" and others wouldn't. But they are all very passionate about what they believe.
Getting back to topic.......Politics aside, I think one of the other crucial factors in the Christian shift in the day of worship is the simple fact that Sunday is the day of the resurrection. Many of the early church "fathers" saw every Sunday worship as a reenactment of the resurrection -- a little Easter of sorts. John says he received his vision of the Revelation on "the Lord's day" (Rev. 1:10)-- a reference most scholars agree is a reference to Sunday (as opposed to the Jewish sabbath). There is a lot of discussion about when John wrote, anywhere from 60 - 95 CE, but it means that many Christians were already seeing Sunday as their holy day in the first century.
Peace, DH