Here I will consider how the MFN theory was expanded to its present bloated proportions. Recently members of the Court, and sometimes a majority, have developed variants of MFN that are far more far-reaching and skeptical than the modest heightened scrutiny suggested by Lukumi.
The key innovation, pioneered by Justice Gorsuch in his concurrence in Masterpiece Cakeshop, Ltd. v. Colorado Civil Rights Commission (a case that did not mention MFN at all) and pursued further in his opinions on Covid vaccination, attributes to the challenged law a different purpose than it actually has, and then declares that the state has discriminated by carving out a secular but not a religious exception to that purpose. Call this MFN-2.
Masterpiece Cakeshop involved a challenge to an antidiscrimination statute by Jack Phillips, a baker who refused to make a cake for a same-sex wedding. Gorsuch thought that religious bias was revealed by a second set of cases that arose at about the same time.