Well, by what 'authority' was the Law of God changed and Sunday substituted as the day of worship, and some may think it has nothing to do with the 'Mark' or 'Number' 666, but it does. The number 666 is the number of a man, and that man is the 'man of sin' spoken of by Paul in 2 Thessalonians 2:3. Revelation 13:18 is where you find 666 and it is associated with that man who claims he has the power of God on earth and is infallible as we see..Seventh-day Adventists teach that the near the end of time the "mark of the best" of Revelation 14 will be placed upon those who worship on Sunday instead of Saturday.
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In his encyclical, "The Reunion of Christendom" (1885), Pope Leo XIII stated that the pope holds "upon this earth the place of God Almighty."
In the Catechism, can be found the following: "The Pope takes the place of Jesus Christ on earth...by divine right the pope has supreme and full power in faith and morals over each and every pastor and his flock. He is the true Vicar of Christ, the head of the entire church, the father and teacher of all Christians He is the infallible ruler, the founder of dogmas, the author of and the judge of councils; the universal ruler of truth, the arbiter of the world, the supreme judge of heaven and earth, the judge of all, being judged by one, God himself on earth." and also "The Pope can make and unmake laws for the entire Church; his authority is supreme and unquestioned. Every bishop, every priest, every member of the Church is subject to him."
Here are some others…"The Pope is not simply the representative of Jesus Christ. On the contrary, he is Jesus Christ Himself, under the veil of the flesh." (Evangelical Christendom, January 1, 1895, pg. 15, published in London by J. S. Phillips)
"Against this background of love towards Holy Church, 'the pillar and bulwark of the truth' (1 Tim 3:15), we readily understand the devotion of Saint Francis of Assisi for 'THE LORD POPE', the daughterly outspokenness of Saint Catherine of Siena towards the one whom she called 'SWEET CHRIST ON EARTH'." (Pope John Paul II, Apostolic Exhortation on the Consecrated Life and Its Mission in the Church and in the World, to the bishops and clergy, religious orders and congregations, societies of apostolic life, secular institutes, and all the faithful, given in Rome, at Saint Peter's, March 25, 1996)
"The Pope is of so great dignity, and so exalted that he is not a mere man, but as it were God and the vicar of God." (Ferraris Ecclesiastical dictionary)
"All names which in the Scriptures are applied to Christ, by virtue of which it is established that He is over the church, all the same names are applied to the Pope." (On the Authority of the Councils, book 2, chapter 17)
"The Pope and God are the same, so he has all power in Heaven and earth." (Pope Pius V, quoted in Barclay, Chapter XXVII, p. 218, "Cities Petrus Bertanous)
"To believe that our Lord God the Pope has not the power to decree as he is decreed, is to be deemed heretical." (the Gloss "Extravagantes" o.f Pope John XXII *** inter, Tit. XIV, Cap. IV. Ad Callem Sexti Decretalium, Paris, 1685)
"Hence the Pope is crowned with a triple crown, as king of heaven and of earth and of the lower regions." (Ferraris, «Prompta Bibliotheca», 1763, Volume VI, 'Papa II', p.26)
"The Saviour Himself is the door of the sheepfold: 'I am the door of the sheep.' Into this fold of Jesus Christ, no man may enter unless he be led by the Sovereign Pontiff; and only if they be united to him can men be saved, for the Roman Pontiff is the Vicar of Christ and His personal representative on earth." (Pope John XXIII in his homily to the Bishops and faithful assisting at his coronation on November 4, 1958)
"This is our last lesson to you: receive it, engrave it in your minds, all of you: by God's commandment salvation is to be found nowhere but in the Church; the strong and effective instrument of salvation is none other than the Roman Pontificate." (Pope Leo XIII, Allocution for the 25th anniversary of his election, February 20, 1903; Papal Teachings: The Church, Benedictine Monks of Solesmes, St. Paul Editions, Boston, 1962, par. 653)
And claims he has the authority to change what God has set....
Protestants ... accept Sunday rather than Saturday as the day for public worship after the Catholic Church made the change... But the Protestant mind does not seem to realize that ... in observing Sunday, they are accepting the authority of the spokesman for the Church, the pope.
—Our Sunday Visitor, February 5th, 1950.
Of course these two old quotations are exactly correct. The Catholic Church designated Sunday as the day for corporate worship and gets full credit – or blame – for the change.
—This Rock, The Magazine of Catholic Apologetics and Evangelization, p.8, June 1997
The [Roman Catholic] Church changed the observance of the Sabbath to Sunday by right of the divine, infallible authority given to her by her founder, Jesus Christ. The Protestant claiming the Bible to be the only guide of faith, has no warrant for observing Sunday. In this matter the Seventh-day Adventist is the only consistent Protestant.
—The Catholic Universe Bulletin, August 14, 1942, p. 4.
The sign of their authority is in changing the sanctity of the Sabbath from Saturday to Sunday. The Protestant churches accepted Sunday as God’s day of worship because that is what the reformers were taught and they didn’t know any better as they broke away from Rome. So its clear the Beast of Revelation is a symbol of that entity, who of herself claims to have changed the solemnity of the Sabbath from Saturday to Sunday, this is her so-called sign of her power and authority.