Oh absolutely. It would have to retain a purely voluntary status. But of course, theocracies are always possible unless the laws of the State are strong enough to forbid it…. and there’s no revolution to overturn constitutions.
Then again, even in “milder” theocracies that are primarily democratic in government, there’s enough secular elements for a viable society. Still, I prefer a freedom-of-religion without state control but that’s mostly because I was raised in a culture promoting it as a value, so it’s hard to be totally objective.