What horrible offense does Dick commit that leads Silver to tell him to pray?

Dick cut a Bible to make a "black spot".

“The black spot! I thought so,” he observed. “Where might you have got the paper? Why, hillo! Look here, now; this ain't lucky! You've gone and cut this out of a Bible. What fool's cut a Bible?”

“Ah, there!” said Morgan. “There! Wot did I say? No good'll come o' that, I said.”

“Well, you've about fixed it now, among you,” continued Silver. “You'll all swing now, I reckon. What soft-headed lubber had a Bible?”

“It was Dick,” said one.

“Dick, was it? Then Dick can get to prayers,” said Silver. “He's seen his slice of luck, has Dick, and you may lay to that.”