Adam Clarke on Revelation 7:3

Till we have sealed the servants of our God - There is manifestly an allusion to Eze 9:4 here. By sealing we are to understand consecrating the persons in a more especial manner to God, and showing, by this mark of God upon them, that they were under his more immediate protection, and that nothing should hurt them. It was a custom in the east, and indeed in the west too, to stamp with a hot iron the name of the owner upon the forehead or shoulder of his slave.

It is worthy of remark that not one Christian perished in the siege of Jerusalem; all had left the city, and escaped to Pella. This I have often had occasion to notice.