While Justice Kennedy wrote this.Chief Justice Roberts had this reply in which he specifically addressed tax exempt status:
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This is interesting, because Roberts is making the slippery slope argument, and Kennedy is making the anti-slippery slope argument. But, they both seem to share the opinion about where the law
should be in relation to religious groups. So what's stopping them from deciding that way if the issue comes up later? Ultimately, a theoretical remark by the Solicitor General doesn't have the force of a supreme court vote when the time comes.