"(... another beast ...) forced everyone, small and great, rich and poor, free and slave, to receive a mark on his right hand or on his forehead, so that no one could buy or sell unless he had the mark" (Rev. 13:(11),16-17)
"If anyone worships the beast and his image and receives his mark on the forehead or on the hand, he, too, will drink of the wine of God's fury .. He will be tormented with burning sulfur ... for ever and ever ... There is no rest day or night for those who worship the beast and his image, or for anyone who receives the mark of his name." (Rev. 14:9ff)
These verses have puzzled Christians for centuries, and this mark has been variously identified as Social Security numbers, Sunday worship rather than a Saturday Sabbath, being part of the Roman Catholic church, and so on. The severity of consequences resulting from the acceptance of this mark make it imperative that we ensure that we have not taken and do not take this mark.
A first step in understanding this mark is to notice the mark's connection to worship of this beast. Whatever or whoever this beast is, he usurps the worship due to Jesus Christ, who is Himself the Almighty YHWH God. Isaiah 43:11 tells us there is no Savior other than YHWH, who in the New Testament reveals Himself as Jesus Christ of Nazareth (Acts 4:12). He does not allow His glory to go to anyone else! (Isaiah 42:8). Any pretender to the throne, anyone or anything that distracts our allegiance from Jesus Christ, is a potential antichrist (1 John 2:18). The believer's defense is to "remain faithful to Jesus" (Rev. 14:12).
Whether we regard Revelation as past, present, or future, or as symbolic or literal, it is significant that BEFORE the beast marks his people, God marks His! In Rev. 7:3, judgment is withheld pending God's mark. Then in Rev. 9:4, those with God's mark are specifically exempted from another judgment, the judgment of locusts. Although it is not explicitly stated, I believe they are exempt from all God's judgments in Revelation.
God's mark is also referred to in the eighth and ninth chapters of Ezekiel. Here, God shows Ezekiel the idolatry and violence of the nation. God then marks His people - those who "grieve and lament" over the sins of the nation. As in Revelation, those with God's mark are spared His judgment which follows.
The Christian today has also been marked - "marked in him with a seal, the promised Holy Spirit" (Eph. 1:13). This is our guarantee of redemption, our exemption from condemnation. Whoever or whatever the Beast of Revelation is, and whatever his Mark is, God has already marked us, and we need not fear the Mark of the Beast. For us, there is now "no condemnation" (Rom. 8:1). Rather than fear, we should live a life of rejoicing!