According to SDA teaching, the mark of the Beast is Sunday worship. The NT does not refer to a Sunday sabbath. There is no NT sabbath at all.
Regardless of denomination and traditional teachings.....
Let's see.
One group of people mostly SDAs say that law will slowly be placed on people to observe Sunday as a day of rest.
They have been predicting this for many years but have never set a day or date.
They believe that the world will be divided into two classes.
Those that keep ALL the commandments of God and have the faith of Jesus.
And those that follow the beast and his commands and teachings..
God's commandments vs Man's commandments.
Rev 14:9 KJV And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,Rev 10 The same shall drink of the wine of the wrath of God, .....
Throughout history the Catholic church was always seen as the beast and antichrist. All the reformers identified the Roman church as the Antichrist.
The Roman Church has gone into a state of changing the thinking and diverting the attention off itself for the last 200 years. Today she still calls herself the the mother church and has not changed.
She killed people for reading the Bible in the dark ages.
The Roman Church says that Sunday worship is her sign and mark.
If this is all wrong and Sunday means nothing (which is what you think) then so be it..
But if laws are enforced to make Sunday the day of rest then you should reconsider your stance.
It is one of many interpretation of what mark of the beast is. Which One is right

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It is a prophecy that I believe is being fulfilled slowing but surely..
Rev 14:12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
Rev 12:17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.