So your last paragraph is not entirely true. According to most Christian doctrines, the tribes of Israel stopped being a thing with Jesus, and the title now referred to the followers of the church created by Jesus. Second, the 144k is less a concrete number and more a representation of perfection. People considered pure good would recieve the seal of god on their forehead to contrast those of pure bad that recieve the mark of the beast. Third, the 144k are given the role to recruit others to fight with god against the antichrist and the beast. Finally, in Catholic and Orthodox doctrine, the 144k are theorized to be the saints reborn to lead tge rightcheous to victory.