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.
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.
---
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!
So let's consider some numbers for effective fertility. Imagine it was 4. That means each generation (which we'll call 30 years, even though it was probably close to 20) would be twice as large as the one prior. And this could be reversed (to see how big your parents' generation was) by multiplying by 0.5. So let's look back to see what the population would have been 2100 years ago, or 70 generations, using various effective fertility rates.
---
4 effective fertility => 8 billion people today * 0.5 (reversible fertility factor) ^ 70 (total generations) = global population in 100BC of... 0!
3 effective fertility => 8 billion * 0.66^70 = global population in 100BC of... 0!
2.5 effective fertility => 8 billion * 0.8^70 = global population in 100BC of... 1316.
---
So we can say that effective fertility rates had to be lower than 2.5 on average! Average being important since we probably went through cycles of higher and lower effective fertility, but that only makes this scarier. There were probably times in the past where humanity's effective fertility dipped below 2, which means we're on our way to extinction. Anybody seeing this, and thinking about the future of humanity, would have realized how fundamentally critical to our survival simply producing more children was. And what's the lowest hanging fruit there? People having non-reproductive sex.