The True Sabbath, and the Mark of the Beast
But Christians of past generations observed the Sunday, supposing that in so doing they were keeping the Bible Sabbath; and there are now true Christians in every church, not excepting the Roman Catholic communion, who honestly believe that Sunday is the Sabbath of divine appointment. God accepts their sincerity of purpose and their integrity before Him. But when Sunday observance shall be enforced by law, and the world shall be enlightened concerning the obligation of the true Sabbath, then whoever shall transgress the command of God, to obey a precept which has no higher authority than that of Rome, will thereby honor popery above God. He is paying homage to Rome and to the power which enforces the institution ordained by Rome. He is worshiping the beast and his image. As men then reject the institution which God has declared to be the sign of His authority, and honor in its stead that which Rome has chosen as the token of her supremacy, they will thereby accept the sign of allegiance to Rome-"the mark of the beast." And it is not until the issue is thus plainly set before the people, and they are brought to choose between the commandments of God and the commandments of men, that those who continue in transgression will receive "the mark of the beast.
- Ellen White, The Great Controversy, p. 449
When Sundaykeeping will be enforced by law (with a death decree) it will become the mark of the beast.
Sundaykeeping is not yet the mark of the beast, and will not be until the decree goes forth causing men to worship this idol sabbath. The time will come when this day will be the test, but that time has not come yet.
- Ellen White, SDA Bible Commentary, vol. 7, p. 977
Once we know what the mark of the beast is, there will be no excuse.
For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
- Hebrews 10:26-27 (KJV)
I strongly suspect that all (or most) of this will be explained during the Latter Rain.
This event is far closer than most think.