Clearly the government can declare them non-essential, and the highest body that arbitrates such claims has rules that California's implementation of the determination was equitable & constitutional. It is defacto legal, allowed, constitutional.

It seems what you take issue with is the definition of "essential". That's fine: but even if California is allowing businesses that are not truly essential to remain open, that doesn't change whether or not Churches are essential. A mistake by California in one are does not require that it apply the mistake elsewhere. And again: every type of gathering style similar to what would be experiences in churches has also be put on hold. Are you aware of any business that has a similar exposure profile as churches, and is not necessary for normal everyday survival (food etc.) that has been allowed to remain open? (Though even if you are, again such a mistake by California does not require it to double down on that mistake with other such gatherings.)