The First Beast has his mark, but those “whose names are written in the book of life” are also marked, or rather, sealed:
“…saying, “Do not harm the earth or the sea or the trees until we have sealed the bond-servants of our God on their foreheads.” And I heard the number of those who were sealed, one hundred and forty-four thousand sealed from every tribe of the sons of Israel:
from the tribe of Judah, twelve thousand were sealed, from the tribe of Reuben twelve thousand, from the tribe of Gad twelve thousand…
…from the tribe of Zebulun twelve thousand, from the tribe of Joseph twelve thousand, from the tribe of Benjamin, twelve thousand were sealed.” (7.3-5, 8)
Contrast 9.4:
“They were told not to hurt the grass of the earth, nor any green thing, nor any tree, but only the men who do not have the seal of God on their foreheads.”
The seal of Rev chapter 7 is St John’s version of this, in Ezekiel 9:
“
1Then I heard Him call out in a loud voice, saying, “Draw near, O executioners of the city, each with a weapon of destruction in hand.”
2And I saw six men coming from the direction of the Upper Gate, which faces north, each with a weapon of slaughter in his hand. With them was another man clothed in linen who had a writing kit at his side. And they came in and stood beside the bronze altar.
3Then the glory of the God of Israel rose from above the cherubim, where it had been, and moved to the threshold of the temple. And He called to the man clothed in linen who had the writing kit at his side.
4“Go throughout the city of Jerusalem,” said the LORD, “
and put a mark on the foreheads of the men sighing and groaning over all the abominations committed there.”
5And as I listened, He said to the others, “Follow him through the city and start killing; do not show pity or spare anyone!
6Slaughter the old men, the young men and maidens, the women and children;
but do not go near anyone who has the mark. Now begin at My sanctuary.”
So they began with the elders who were before the temple.”
The “mark” in Ezekiel 9 is the letter Taw, the final letter of the Hebrew alphabet. The Greek letter Tau, which is related to it, is, like it, approximately cross-shaped. If, as is hardly doubtful, Rev 7 is St John’s updated version of Ezekiel 9.1-6, it seems likely that the “seal” of Rev 7 is either the Cross, or Baptism, which the Fathers called “the seal”.
Rev 9.4, with its comment on the servants of God with “the seal of God on their foreheads”, prepares for, and is parodied by, those who have “the mark of the beast…on their right hand
or on their forehead”.