"On the sixth day of the week, and the sixth hour, says St. John, the kingdoms of Christ and Antichrist looked one another in the face in Pilate's court, and the adherents of the False Prophet (Caiaphas) firmly wrote on their foreheads the mark of the Beast, when they said, 'We have no king but Caesar'... Christ's Friday victory is the supreme manifestation also of the Antichrist."
-- A. Farrer, A Rebirth of Images: The Making of St. John's Apocalypse, p. 259
With six words, these folks made a decisive pledge of allegiance and so received the mark of the beast. With three words, "Jesus is Lord," the Jews of the seven churches of Asia Minor would pledge allegiance to Christ and receive His mark. This is essentially Farrer's thesis: to receive a mark means to pledge allegiance.
Modern Christians are fascinated by the mysterious mark of the beast. But misinterpretation can grow out of our neglect for historical context when we treat the Bible as if it were a golden story book that simply dropped from heaven. Now, if any book of the Bible could be said to have dropped straight out of heaven, it's Revelation. Yet God unveiled his heavenly plan using earthly imagery. Using scripture and other ancient sources to interpret scripture can help us to have a historically and culturally grounded understanding of the connection between marks and allegiance in the ancient world. To paraphrase Ben Witherington, a text without context is just a pretext for whatever you want it to be.
I can come up with six sources which I think support Farrer's thesis that to receive a mark means to pledge allegiance.
First, the mark or seal is a dominant theme of the book of Revelation, but this passage should give an idea of the seal of Christ. Revelation 7:1-4:
"After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth so no wind could blow on the earth, on the sea, or on any tree. Then I saw another angel ascending from the east, who had the seal of the living God. He shouted out with a loud voice to the four angels who had been given permission to damage the earth and the sea: 'Do not damage the earth or the sea or the trees until we have put a seal on the foreheads of the servants of our God.' Now I heard the number of those who were marked with the seal, one hundred and forty-four thousand, sealed from all the tribes of the people of Israel."
Second, here's a contrasting mention of the Beast's mark. Revelation 14:9-10:
"A third angel followed the first two, declaring in a loud voice: 'If anyone worships the beast and his image, and takes the mark on his forehead or his hand, that person will also drink of the wine of God’s anger...'"
Third, we have extra-canonical historical precedent for a Jewish idea of sealing by mark specifically on the forehead. Pss. Sol. 15:6,9:
"The mark of God is on the righteous so that they may be saved.... the mark of destruction is on their [sinners] forehead."
Fourth, the precedent goes way back. Ezekiel 9:4-6:
"The Lord said to him, 'Go through the city of Jerusalem and put a mark on the foreheads of the people who moan and groan over all the abominations practiced in it.' While I listened, he said to the others, 'Go through the city after him and strike people down; do no let your eye pity nor spare anyone! Old men, young men, young women, little children, and women – wipe them out! But do not touch anyone who has the mark.'"
Fifth, if you need a non-Jewish reference, check out Herodotus' Histories 2.113 where he is discussing the story of Paris (of Trojan War fame) and how, after stealing Helen from the Spartans, had blown off course toward the coast of Egypt:
"On the shore there was - and in fact there still is - a sanctuary of Herakles to which anyone's servant may flee for asylum, have himself branded with sacred marks, and devote himself to the service of the god. This custom has been in effect since the beginning, and it is still the rule in my time."
Sixth, a more oblique reference, but for me, it's the clincher. Exodus 12:3-13:
"Tell the whole community of Israel, ‘In the tenth day of this month they each must take a lamb for themselves according to their families... Your lamb must be perfect, a male, one year old; you may take it from the sheep or from the goats. You must care for it until the fourteenth day of this month, and then the whole community of Israel will kill it around sundown. They will take some of the blood and put it on the two side posts and top of the doorframe of the houses where they will eat it. They will eat the meat the same night... I will pass through the land of Egypt in the same night, and I will attack all the firstborn in the land of Egypt, both of humans and of animals, and on all the gods of Egypt I will execute judgment. I am the Lord. The blood will be a sign for you on the houses where you are, so that when I see the blood I will pass over you, and this plague will not fall on you to destroy you when I attack the land of Egypt.'"
With the lamb's blood, God was essentially providing a mark for his people. All they had to do was choose to apply the mark. In Egypt the whole community of Israel killed the perfect lamb around sundown. In Pilate's court, the whole community of Israel killed the perfect lamb around sundown.
He took the bread, broke it, gave it to his disciples and said, "This is my body."