Do you keep Saturday (the Sabbath) holy?
That rule, in particular, means nothing if it wasn't given by God, does it? Why should I care what I do on Saturday (or Sunday), if there is no God, and all of this is just some "fictionalized account" that teaches us some truth.
(The New Testament permits worship on Resurrection Day...Sunday. Also, Jesus pointed out that most of the little legalisms that were practiced on the Sabbath were entirely unnecessary. The Sabbath was supposed to be renewing and refreshing for humans -- not a pain in the ass.)