Scenario.  The politicians of Earth finally did it, and the nukes went flying.  One hit the wrong stuff, and the resulting radiation half life on Earth is sixty years.  

The only survivors were 24 people who got off in a spaceship just in time.  Then there was a disease that killed off everyone on that ship except you and your sister.  

As far as you know, only you two are alive.

Does the race die out because you follow The Book, or do you "just make up some words that justify it?"

Although this is a made up scenario, you can see clearly that it overrides rules made up and put down in an old book and alleged to be "The Word of God."  

FAR MORE COMPLEX QUESTION.  One of the two left living wants to, the other absolutely refuses.