I do not know, but the law seems to treat religion as less important. If a law is not aimed at speech, but hits communicative conduct (law outlaws destruction of a draft card which might or might not be done in the presence of another), the law can be constitutionally applied only if it furthers a substantial state interest by means no greater than is essential to the furtherance of that interest. But if a statute prohibiting particular conduct (eg., ingesting peyote) whether religious or not religious, hits religion (e.g., a Native American religious ceremony involving the ingestion of peyote), no First Amendment test applies unless another constitutional right is also burdened or the government is singling religion out for special treatment.
But suppose a First Amendment test is warranted in a statute directed at the content of a communication or in a statute primarily aimed at a form of religious conduct. In the speech context the courts require the government to show that the statute or regulation is necessary to further a compelling state interest. This is called “strict scrutiny.” On the other hand, if a statute is directed against religious conduct, the courts apply a less demanding standard to the government action even though the courts will label their standard as “strict scrutiny.”
The courts have not explained the basis for this different treatment. Perhaps they think the justification is obvious. But I think you have to get up very early in the morning to explain the different treatment of speech and religion – particularly when these rules have been fashioned by judges who are members of religious associations. Although I am writing this post very early in the morning, I have to say I am baffled.