The traditional Christian faith continues to get itself in a metaphorical box when it claims there is life after death. Since not one person can claim to know the experience, the door is open to every kind of phony arguments. The phoniest one starts with the Christian theory there is actually life when we are dead as stones.
The Mormons took the orthodox Christian concept of life after death and expanded on it. They decided that if there were all these non Mormon souls floating around, they would save them from hell by converting them to their own version of Christianity. They held ceremonies baptizing dead people.
The link points out Mormons do not believe they are forcefully converting people against their will. The dead can decide for themselves:
Each individual (has)…the right to choose the validity of a baptism for the dead. It depends on the deceased person accepting it and choosing to accept the Savior while residing in the spiritual world.
Thus, dead people are considered fair game for Mormons (though officially the Mormon Church no longer sanctions the practice). Why could not the dead be converted to Islam or Hinduism. So far as I know nothing in the Bible says the afterlife is off limits for recruitment by religions.
When the ancients wrote in the Bible about heaven and hell they thought they had invented a foolproof carrot and stick concept that only they could manage. Little did they know modern religions like Mormonism could steal their cake.