Hi, Post Tribber!
I note that you are a fellow Protestant, though not an Adventist. It is hard to understand what we are talking about if you are not very familiar with our theology, escathology, and our understanding of church history.
When you mention that the majority of Christian faiths are Sunday-keeping faiths, you may be surprised to hear that this was not always the case. The apostolic churches were all Sabbath-keeping churches. When Rome destroyed Jerusalem in AD 70, they expelled all Jews from Jerusalem, and this included the leadership of the Church of Jesus Christ, because most of them were Jews, and all of them kept the Sabbath. When the headquarters of the church was moved to Antioch, the hub of Christian missionary outreach to Asia, a rival headquarters began to emerge in Rome itself. The Western church, which was influenced by the Church Fathers in Rome and Alexandria, wanted to distance themselves from the Jewish heritage of Christianity. They also wanted to escape persecution by Pagan Rome. So the Western church began to incorporate in their Christian belief and practice, some sacraments and doctrines which were influenced by Paganism. One of them was to change the day of worship from the Seventh-day Sabbath to the Roman Dies Solis, or “Day of the Sun.” Meanwhile, the churches in the East, which later became known as the Eastern Orthodox Church, kept the Sabbath, and kept to more of the Biblical teachings of the apostles. The church in the West recognized Rome as the most important center of world government, culture and religion, and so they regarded the Bishop of the church of Rome to be the most important of all Christian leaders. The Roman Empire accepted Christianity as a legal religion (with certain stipulations) by the Emperor Constantine. Constantine made Sunday the official day of worship of the Western church. Now, Pagan Rome became Christian Rome, or Papal Rome, as the Bishop of Rome was elevated to the position of leader of the church in the west. One of the Pope’s first acts was to excommunicate the Patriarch of the Eastern church, because he wanted to be the exclusive, unchallenged leader of the Christian church, and to control and enforce all church doctrine and practice. Two of the Pope’s early conflicts with the Church of the East were the proper date of the observance of “Easter,” which the Eastern Church celebrated on the first Sunday following the Jewish Passover (as opposed to the Western Church, which took the first Sunday after the Vernal Equinox- which coincidentally was the worship day of the Pagan goddess of fertility, Ishtar- where we get the word “Easter” from), and the observance by the Eastern Church of the Seventh-day Sabbath, as opposed to the Sunday, honored in the West. The conflict became very bloody indeed with centers of the Inquisition being set up all over Asia, most notoriously in India, to force the Sabbath-keeping Christians into submission. It eventually resulted in the almost complete eradication of Sabbath keeping and it’s memory in the Christian world. The tide was turned in around the seventh century. Previous to that time, the majority of Christians were Seventh-day Sabbath-keepers. Basically, Rome changed Sabbath to Sunday. Sunday remains a mark of the Pope's eccliastical authority over Christianity.
The Protestant Reformers, seeking to re-establish the Biblical principles of Righteousness by Faith, wanted to re-create in Christianity the pure Biblical doctrine. But the Seventh-day Sabbath had become so identified in the minds of Christians with legalism and works righteousness, that the Reformers never had either the political capital, nor “teachable moment” to restore this Biblical truth and commandment of God to Christianity. Moreover, Europe was anti-Semitic. So. Sunday Remains!