The Seal Of God was a three-dimensional type cross mark that appeared on the foreheads of believers from the point just before the first Trumpet Judgment until the glorious appearing of Jesus Christ. Believers could see this mark on the foreheads of other believers but cannot see their own mark even in a mirror. It also appeared on the foreheads of the 144,000 Jews that were called and sent to evangelize throughout the whole world. Those who bore the Seal Of God were immune to the effects of the fifth and sixth Trumpet Judgments well as from the third, fourth, fifth, and seventh Bowl Judgments. However, they could still be caught and martyred, though Tsion Ben-Judah promises that those sealed by God cannot and will not be able to change their minds about their standing with God when it comes to making the ultimate life-or-death decision during the latter half of the Tribulation.
Effectively, it works somewhat like the "mark of loyalty" system that Carpathia instantiates, though those that took the mark could express regret about taking it and realize that their decision is irrevocable.