Just because something's a spinoff doesn't mean it's unoriginal.There are so many spinoffs of Judaism.
Christianity was the first, changing the teaching of obedience to the law to one of belief, and making a man into God (at least the orthodox version).
Then Islam came, a spin off of a spin off.
Most recently we have Baha'i, a spin off of a spin off of a spin off.
All of them change the original message.
Why should I take all these spin offs seriously when I have the original? Which would you rather have: a copy of a Van Gogh, or the original?
I mean, Christianity has about as much to do with Judaism as Mork & Mindy has to do with Happy Days.