This should give you an idea of their mindset.
“Sunday-keeping must be the mark of the beast.” … “The reception of his mark must be something that involves the greatest offense that can be committed against God.” (The Marvel of Nations, Elder U. Smith pages 170, 183)
“Here we find the mark of the beast. The very act of changing the Sabbath into Sunday, on the part of the Catholic church, without any authority from the Bible.” (Ellen G. White, The Mark of the Beast, page 23)
“The Sunday Sabbath is purely a child of the Papacy. It is the mark of the beast.” (Advent Review, Vol. I, No. 2, August, 1850.)
“The change of the Sabbath is the sign or mark of the authority of the Romish church.” … “The keeping of the counterfeit Sabbath is the reception of the mark.” (Ellen G. White, Great Controversy, Vol. 4, page 281.
“The mark of the beast is Sunday-keeping. A law will enforce this upon Seventh-day Adventists. They won’t obey. Then they will be outlawed, persecuted, and condemned to death! Of all the wild Advent speculations in the prophecies, this deserves to stand among the wildest.” (Seventh-day Adventism Renounced" by D.M. Canright, 1914)
“Sunday-keeping is an institution of the first beast, and ALL who submit to obey this institution emphatically worship the first beast and receive his mark, ‘the mark of the beast.’ … Those who worship the beast and his image by observing the first day are certainly idolaters, as were the worshippers of the golden calf.” (Advent Review Extra, pages 10 and 11, August, 1850)
bible.ca/7-mark-beast.htm
Go here, and search on words like babylon, catholic, pope, papacy, etc., to see for yourself what they think.
http://egwdatabase.whiteestate.org/nxt/gateway.dll?f=templates$fn=default.htm$vid=default
http://egwdatabase.whiteestate.org/nxt/gateway.dll?f=templates$fn=default.htm$vid=default