Watchtower Simplified Study Edition, March 2015, Page 22:
13 Just before the great tribulation, God will give his final approval to the hardworking anointed ones who are still on earth at that time. This is their final sealing. (Revelation 7:1-3)
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Watchtower Study Edition, March 2015, Page 15:
11 So, then, before the great tribulation starts, all the faithful anointed on earth will have received their final sealing. (Rev. 7:1-4) From then on, their calling is sure. But think about the years before the tribulation begins. What would happen to anointed ones who failed to remain watchful, who lost their integrity? They would lose their heavenly reward. Obviously, they would receive no final sealing prior to the start of the tribulation. By that time, other faithful ones would be anointed. When the tribulation starts, the foolish ones might be shocked to see the destruction of Babylon the Great. It may be only at that point that they realize that they are not ready for the arrival of the Bridegroom. At that late hour, if they in desperation ask for help, what would happen? Jesus’ parable gives us the grim answer. The discreet virgins refused to give up their oil for the sake of the foolish, telling them to go instead to the sellers of oil. Remember, though, it was “in the middle of the night.” Would they be able to find sellers of oil at that hour? No. It would simply be too late.