The Mark of the Beast (In Short)
The seal of God, and the mark of the beast are both a sign of who you worship.
- Worship him that made heaven, and earth, and the sea, and the fountains of waters (Revelation 14:7)
- Worship the beast and his image (Revelation 14:9)
The Sabbath is a sign you worship God (obedience, the Ten Commandments).
Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it is a sign between me and you throughout your generations; that ye may know that I am the Lord that doth sanctify you.
- Exodus 31:13 (KJV)
The Sunday is a sign you worship satan (rebellion, commandments of men).
But in vain they do worship me, teaching for doctrines the commandments of men.
- Matthew 15:9 (KJV)
It was the Roman Catholic Church who changed the commandments, e.g. Sabbath to Sunday.
Of course the Catholic Church claims that the change was her act. And the act is the mark of her ecclesiastical power and authority in religious matters.
- Faith of Our Fathers, p. 14, C.F. Thomas, Chancellor of Cardinal Gibbons
You will either live with God in obedience (seal), or you will die with satan in rebellion (mark).
Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man.
- Ecclesiastes 12:13 (KJV)
The mark of the beast = knowingly keeping Sunday instead of Sabbath.