THE SABBATH IS THE SEAL OF GOD.
The sign, or seal, of God is revealed in the observance of the seventh-day Sabbath, the Lord’s memorial of creation.... The mark of the beast is the opposite of this—the observance of the first day of the week.—Testimonies for the Church 8:117 (1904). – {LDE 224.3}

When the test comes, it will be clearly shown what the mark of the beast is. It is the keeping of Sunday.—The S.D.A. Bible Commentary 7:980 (1900). – {LDE 224.2}byhy