Now then. Where to begin.. in the context of real time I havent seen the film a large amount of times but I can appreciate exactly what its is. Anyway I personally have read all manner of fan theorys behind what can possibly reside with the case which is being 'protected' by Vince and Jules from actual literal gold to whatever but I think that in Jules case ( Pardon the pun) the contents is of higher power a sort of physical manifestation of the lord if you will thats why he chooses to leave his mob life behind and "walk the earth"