what does gandalf say to suggest that his reason for involving bilbo in the aadventure is more than a joke?

Bilbo faints after the quest (and the fact that he will be a part of it) is mentioned. Needless to say, the dwarves think the prospect of him coming along or even being able to assist them is a joke. Gandalf, on the other hand, has his own ideas, and he's the boss. His response to the skeptical;

“Let’s have no more argument. I have chosen Mr. Baggins and that ought to be enough for all of you. If I say he is a Burglar, a Burglar he is, or will be when the time comes. There is a lot more in him than you guess, and a deal more than he has any idea of himself. You may (possibly) all live to thank me yet.”