Why do Seventh-day Adventsists believe in a great end of days conflict about the Sabbath day?

The Biblical predictions refers to the predictions found in Revelation of the reunification of Church and State, specifically in America, in the last days.

Below is what is believed by most Seventh Day Adventists:

In Rev 12:13-16, we see that God's church (the women) was a persecuted church throughout history. She existed through a time where there was no liberty of conscience. She was hidden in the wilderness for "a time, times, and half a time" (Rev 13:14) to save her from being "carried away of the flood" from the dragon (Rev 13:14), until the "earth the helped the women" (Rev 13:16).

Now water in Revelation refers to people "The waters which thou saweth, where the who*e sitteth, are peoples, and multitudes, and nations and tongues" (Rev 17:15). Conversely, earth would mean a land relatively uninhabited. The time period, "A time, times and half a time" is 1,260 days (Rev 12:6), and refers to 1260 years if a day in prophetic time equals a year. Combined it depicts: God's church persecuted and hidden for 1,260 years, until the reformation during the later period. A large number of protestants sailed from Europe to a new land, North America, founded on religious liberty.

Next, Revelation 13 describes two beasts, the beast from the sea (Rev 13:1-10) and the beast from the earth (Rev 13:11-17).

The beast from the sea is the persecuting power that rose out of the Roman empire. Those familiar with Daniel 7 would tie the description of the beast in Rev 13:2 to be the cumulation of the empires described in Daniel 7. It is also a religious power as the beast is actively worshipped (Rev 13:4) and speaks blasphemies (Rev 13:5). It rules as a civil power for 42 months, which is the 1,260 year period in History mentioned (Rev 13:5). It made "war with the saints" of God and had civil authority as "power was given him over all kindreds, and tongues, and nations" (Rev 13:12). This is the church of Rome, the Roman Catholic church of the middle ages, which became corrupted from within.

Lastly, we get to the beast from the earth. The earth already points strongly to North America, a land that was relatively uninhabited (Rev 12:16). The beast that comes out of the earth has two horns like a lamb, which points to America's two founding principals as the reason for its power -- civil and religious freedom. However this beast would speak like the dragon, infringing on religious freedom. Revelation predicts that the civil government of the United States reunites with world loving Christian religious leaders, slowly removing the separation of church and state.

Revelation 13:12-14, describes predictions that have not yet happened. Prophecy points to a time when God will allow those working for the dragon to exercise false miracles (Rev 13:13-14), and all who do not have a strong foundation in the Bible will be deceived, no matter the denomination. World loving churches will reunite with the papacy and worship the first beast (Rev 13:12). Rev 13:14 describes a democratic power as it appeals to the people, and the image of the beast is the style of government similar to the first beast that forces religious conscience (Rev 13:15). However, all who love God, of every denomination, will come out of her (Rev 18:4).

Under this environment the worship of God on Sunday becomes a topic of controversy. It is the only law of the ten commandment that was changed by the papacy. The Catholic church said so itself as evidence of its power from God. Efforts will be made by the beast to pass Sunday worship as law, just as in history. Only then does Sunday worship become the mark of the beast as described in Rev 13:15-16.

For further study on this topic, I recommend whole heartily the book "The Great Controversy" by Ellen White (Wikipedia, Amazon, Goodreads), written almost 100 years ago, but even more relevant today.

This idea primarily comes from the teachings of Ellen G. White, whom the Seventh-day Adventists consider a modern-day prophet.

Early Writings, p 65:

The pope has changed the day of rest from the seventh to the first day. He has thought to change the very commandment that was given to cause man to remember his Creator. He has thought to change the greatest commandment in the decalogue and thus make himself equal with God, or even exalt himself above God. The Lord is unchangeable, therefore His law is immutable; but the pope has exalted himself above God, in seeking to change His immutable precepts of holiness, justice, and goodness. He has trampled underfoot God's sanctified day, and, on his own authority, put in its place one of the six laboring days. The whole nation has followed after the beast, and every week they rob God of His holy time. The pope has made a breach in the holy law of God, but I saw that the time had fully come for this breach to be made up by the people of God and the waste places built up.

This is based on her interpretation of Daniel 7:25 (ESV):

He shall speak words against the Most High, and shall wear out the saints of the Most High, and shall think to change the times and the law; and they shall be given into his hand for a time, times, and half a time.

In the same vein as many Premillenial Dispensationalists, White identifies the 4th beast in the above passage as the Antichrist who will appear before a great tribulation prior to the return of Christ and a subsequent 1000-year reign.

But although Adventists are not the first regarding the interpretation of the Papacy being identified as the Antichrist (prominent protestants, such as Luther, Calvin, Knox, and others have held that position), they do understand this in the context of what they call the Great Controversy (more on this below).

The important phrase relevant to the question is "shall think to change times and the law;" which is understood to specifically by the Seventh-day Adventists to mean changing Saturday worship (one of the Ten Commandments) to Sunday worship.

There are plenty of SDA books regarding the Sunday/Sabbath conflict, the most important being White's "The Great Controversy between Christ and Satan", published in 1911. There she also talked about the role of the United States of America in the final conflict (identifying it as the Second Beast of Revelation 13) . This view is also present in other popular, more modern books, like Jan Marcussen's, National Sunday Law.

Some background facts which make this an especially important point to the Seventh-day Adventists:

1. Spirit of Prophecy

Ellen G. White is regarded as the modern day SDA prophet having the "spirit of prophecy" referred to in Rev 19:10. See also Rev 12:17.

This leads the SDA church to regard themselves as a special "last days" remnant (because only they properly keep the commandments) and having their own special revelation (Ellen White's writings).

2. The Great Controversy

The Great Controversy is a very important theme in the SDA church. National Sunday Law (the pope changing the day of worship from Saturday to Sunday) is an acute representation of this larger conflict. According to the SDA church, Sunday keeping will be forced by civil laws all over the world, separation of State and Church will be lost (even in the USA), and the Papacy will lead the final movement against God's Law.

3. Sabbath Day

Although the Ten Commandments are all of equal importance for the SDA Church, keeping the Sabbath holy is considered of special importance during the final conflict, due to the following reasons:

• It's the only Commandment of the Ten that is not universally shared by other Christians, not even Protestants. That will make the Sabbath very difficult keep in the final context described in the Great Controversy.

• According the SDA beliefs, God's law must be kept as a whole (whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it - James 2:10). So Sabbath can not be negotiated, not even for the sake of union with other Christian denominations.

For the SDA church, the mark of the Beast during the final days (Rev. 13:16-17) is Sunday's observance, as a system instituted by men (the Papacy), in contrast to God's real Commandments (Sabbath observance).

Conclusion

Put these three facts together, and you get a great end-times controversy over the Sabbath commandment which the faithful remnant follow because they alone have the "Spirit of Prophecy."