The Christian rule that one should rest on the seventh day grew by bits and pieces. This was never a rule that Jesus set down, as he was not so silly. In the beginning this rule, like many that mankind ends up slavishly following, was expedient for a given locale at a given time. The workers were burning out, and needed guidelines. This could be equated to a union steward insisting that the 40 hour week be maintained. When others heard about this new social benefit and desired this for themselves, they had to put some meat into it and thus placed God behind the order. Since this story benefited those who spread it, the story spread like wildfire.