A Thought About What Makes a Burden "Substantial"

There has been some discussion about what it is that would make a "burden" qualify as "substantial" under the terms of the Religious Freedom Restoration Act.  I had a thought about this that I wanted to try out here.  It seems to me that the gravity of a burden is frequently intimately connected with the centrality, or importance, of the belief that is felt to be burdened: the more central, or important, the religious tenet or view, the greater the obligation that the claimant will feel in adhering to it, and the more substantial will be the burden felt by the claimant in having to endure not adhering to it.  Yet RFRA eliminated the inquiry into centrality.  That decision was, I think, intended to prevent courts from making judgments about (a) how important a belief was, in order to determine (b) how severe or substantial the burden was.  One reason for eliminating the centrality inquiry was an establishmentarian concern; another was a concern about competence.  

The difficulty is that the standard continues to be a "substantial" burden.  That cannot only mean a burden as to which a claimant sincerely objects on religious grounds.  But how would one determine a burden's substantiality without being permitted to inquire at all about a belief or practice's centrality, or importance?  I'm not even sure what the inquiry would look like.  And that may be why, in the RFRA case law, one tends to see a great deal of deference to the claimant about what constitutes a "substantial" burden in the first place (and cases often get resolved under the compelling interest leg) -- exactly because of the danger that an inquiry into the burden's gravity, or substantiality, can easily bleed over into an inquiry about the belief or practice's centrality, or importance, within the religious system.  Sometimes one sees the statement that a substantial burden is one where the state puts "substantial pressure on an adherent to modify his behavior and to violate his beliefs."  Thomas v. Rev. Bd.  But that only seems to restate a kind of subjective test -- how much pressure is "substantial pressure" will depend upon an inquiry about the nature of the coercion felt by the claimant in light of the religious belief's importance to the claimant.  Pressure only matters if the belief is religious (not generally a question) and about something important...or central.  That is, a claimant is sensitive to pressure if government is squeezing a pressure point.  But because centrality is no longer a cognizable concern, we are necessarily left with a healthy measure of deference to the claimant's feelings about the quality of the burden.  Thoughts?