1) An effectively fertility rate of "N" will result in the next generation being N/2 times as large. So a fertility of 2 means the next generation will be exactly the same size (2 people, having 2 kids who have 2 kids...) while a fertility rate of 4 would result in a doubling of size, and a fertility rate of 1 would result in the next generation being half as large.
2) To reverse the multiplication of some value by "X", you simply multiply by 1/X. So if our population increases by 1.5 in a generation, then to reverse that and get from the new population, back to where we started, we'd multiply by 1/1.5 or 0.66. So for instance 100 * 1.5 = 150. And 150 * 0.66 = 100.
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So taking the most complex example. Fertility rate of 2.5 = a population increasing factor of 1.25. Reversing that means multiplying by 0.8
So if our parent's generation had a fertility rate of 2.5, that would mean that their population would be 0.8 times the size of our population, which I take as the global population which is about 8 billion. And if their parents also had a fertility rate of 2.5, then the exact same would follow for another multiplication of 0.8. So if we go back "N" generations, we'd end up with a population, for that generation, of 8 billion * 0.8^N.
So going back 70 generations with a fertility rate of 2.5, we finally get the original formula: 8 billion * 0.8^70 = world population 70 generations ago. You end up with this problem that if you don't have a really low effective fertility rate, the population approaches 0 far too quickly, and that's only going back 2100 years!
And on the flip side, secular society prides itself on how smart, objective, no "sky daddy" fantasies, etc it is as it marches towards self-selected extinction.