Revelation shows that the Sabbath will be a sign of obedience at the end of history. We have to remember, however, that a person’s observance of Sunday now does not itself mean that he or she has the mark of the beast.
Sunday keeping will become “the mark of the beast” only when, having clearly understood the issues involved in choosing a day of worship, despite the deceptions out there, people make their choice either for or against God. However, that time still lies in the future.
“No one has yet received the mark of the beast. The testing time has not yet come. There are true Christians in every church, including the Roman Catholic communion. None are condemned until they have had the light and have seen the obligation of the fourth commandment. But when the decree shall go forth enforcing the counterfeit sabbath, and the loud cry of the third angel shall warn men against the worship of the beast and his image, the line will be clearly drawn between the false and the true. Then those who still continue in transgression will receive the mark of the beast.”—Ellen G. White, Evangelism, pp. 234, 235.
Let us remember that Sunday observance today does not make a person lost any more than Sabbath observance makes a person saved. The time is coming, however, when “the mark of the beast” will become the central issue and when choosing a day of worship will be the test of faithfulness.
Revelation appeals to God’s people to take the Bible and, with a heart-searching spirit, study the prophetic word for themselves and make every effort to reach with the gospel those who are today unreached for Christ.
I’m studying this further here: https://www.amazingfacts.org/media-library/book/e/53/t/the-beast–the-dragon-and-the-woman
Source reference: SDA 1Q19 Quarterly