Suppose, just suppose the Dominionists get what they want. And we live in a theocracy.
And suppose, just suppose they have a major case against someone, and I'm their crown witness. They have a case, except they don't. The defense will shred my testimony down to submolecular level.
Because I declare myself as an agnostic, they have a witness they cannot use.
The crucial moment is when I need to be sworn as a witness, right after the "andnothingbutthetruth, so help me God".
If I lay my hand on the bible and say I do, I have lied already. I don't believe - so the "so help me God" wouldn't be truth in my case. So let's say I refuse to swear on the Bible, and I go with a civilian version, which ends with "andnothingbutthetruth". The defense would have a field day - imagine, Dominionists' main witness is an unbeliever! Somebody they deem a sinner, not trustworthy and lacking morals from the outset. And their star witness refuses their Bible, on the grounds that it wouldn't be truth for him. Ha! What's this guy, trying to show honesty outside any Christian system of belief?
So they decide to run their case without me. The defense gets a free ride - by either building the case on the scarcity of other evidence, or on this omission. They may even take me to be their witness.
Now imagine worst-case scenario: I'm accused. I'm done in advance. I may start googling out recipes for cakes with metal files in them.