"And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name." (Revelation 13:16-17)

The mark will have a three-fold purpose:

First, it will allow the Antichrist to control commerce. Merchants will not be able to buy and sell merchandise and people will not be able to purchase goods without the mark, thereby allowing the Antichrist to control all business.

Second, it will allow the Antichrist to monitor the movements of every person on earth who has the mark. Developments in technology make having this capability very near.

Third, everyone taking the mark will be showing their loyalty to the Antichrist, and by doing this their fate is sealed.

Anyone who takes the mark and worships the 'Image of the Beast' instantly loses their opportunity of eternal life with Jesus.They cannot be saved because you cannot both become a Christian and follow the Antichrist. You have to serve one master or the other, either God or Satan:

"And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:" (Revelation 14:9-10)

You may have heard of the number '666'. This refers to the Antichrist and the mark:

"Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six." (Revelation 13:18)

It's not yet known what this number represents with the beast because the Antichrist has not yet risen to power and been revealed. When he does this number will become self-explanatory.

So, the Christians are raptured to Heaven to be with Jesus who promised to keep them from the "hour of temptation" (Revelation 3:10). So you may be asking, Will Jesus Be Back?