Edge,
In order to be immortal, a soul must exist indefinitely. For example, let us estimate the average Human life span as being 100 years. Just since Christ it has been about 2,000 years, which is 20 full life times. Before that civilized history stretches out about 8,000 years for a total of about 10,000 years in the past. That is about 100 full life times.
If we look at the history of life before Human civilization, it stretches back about 4 billion years, which is 40 million full life times.
There is probably about another 5 billion years before the sun will begin making life on earth impossible, which is 50 million full life times from now. Even then, the Human race, if it is still around, would be capable of moving out of harms way.
But, even 90 million full lifetimes is short compared to all eternity. The point here is to give some small idea of how long an immortal soul would be around.
Does ones immortal soul begin at our mortal birth or does it exist an eternity before our mortal birth?
After our mortal death, what does the immortal soul do for all the rest of eternity?