Is the Will of D a Bloodline or a Belief System?

We’ve seen several D. bearers now, from Gol D. Roger and Monkey D. Luffy to Trafalgar Law and Portgas D. Ace. The series hints at an inherited will tied to the initial D., but I’m starting to wonder if it’s less about blood and more about a shared mindset. Roger and Luffy aren’t related, yet both smile in the face of death and