To understand WHAT the mark of the beast is, you have to ASK the beast themselves what their MARK is...

"The Bible says, Remember that thou keep holy the Sabbath day. The Catholic church says, No! By my divine power I abolish the Sabbath day, and command you to keep the first day of the week. And lo, the entire civilized world bows down in reverent obedience to the command of the holy Catholic church!" Father Enright, C.S.S.R. of the Redemptoral College, Kansas City, History of the Sabbath, p. 802

Question - Which is the Sabbath day?
Answer - Saturday is the Sabbath day.
Question - Why do we observe Sunday instead of Saturday?
Answer - We observe Sunday instead of Saturday because the Catholic Church, in the Council of Laodicea (A.D. 364), transferred the solemnity from Saturday to Sunday." The Convert's Catechism of Catholic Doctrine, p. 50, 3rd ed.

"Of course the Catholic Church claims that the change was her act... And the act is a MARK of her ecclesiastical authority in religious things." H.F. Thomas, Chancellor of Cardinal Gibbons.

"Sunday is our MARK of authority. . .the church is above the Bible, and this transference of Sabbath observance is proof of that fact" Catholic Record of London, Ontario Sept 1,1923.

"It was the Catholic church by the authority of Jesus Christ, which has transferred this rest to Sunday in remembrance of the resurrection of our Lord. Therefore the observance of Sunday by the Protestants is an homage they pay, in spite of themselves, to the authority of the Catholic church." - Mgr, Segur Plain Talk about Protestantism of today page 213

Doesn't take a rocket scientist to figure out what the mark will be, the antichrist tells you what it is... it will be an ENFORCED day of worship, LEGISLATED, a LAW... Sunday of course!! You WILL accept it (symbolically) in your forehead, you agree with it and go along with it (and most main stream religions already keep Sunday as their holy day, so it will come perfectly natural for most Christians (sadly), or in your hand (symbolically), you don't necessarily agree with it or even care about God etc, but you will obey so you can buy, sell, etc.

OR you will have the seal of God and NOT take the mark, and every earthly support will be cut off, making true Christians rely completely on God for everything. It's the first time it's ever happened. Go read about the Israelites when they were getting prepared to leave Egypt... the Exodus. There are a lot of parallels. God's people will also be miraculously delivered after the 7 last plagues fall on humanity that has the mark of the beast, just like the plagues fell on the Egyptians before the Exodus (except it was 10 plagues then).

It's coming... be prepared!