The debt that all men pay was the first clue in National Treasure: Book of Secrets. The answer is death. It was the final words Thomas Gates said to his son Charles and is figured out by his descendant Benjamin Franklin Gates. The word "death" is part of the Playfair Cipher needed to solve where the next clue is Laboulaye Lady.
The Playfair Cipher was never fully decoded. This is the text that was encoded:
"Laboulaye Lady will lead to Cibola temples of gold."