If there is a letter which reveals the existence or absence of God then God MUST EXIST regardless of what the letter says. If God didn't exist then there wouldn't be any such letter in the first place.

What power could possibly claim that God doesn't exist except for God himself? The problem is that in case of a negative answer God is a liar, a terrible one too.

I think a theist opening the letter shows more faith.
Many theists I know would be crushed if they found out with 100% certainty their God is a lie. To the point they already completely ignore everything we know about modern science.

If they truly believe their God is real they should have no issues with opening the the letter with complete faith of it solidifying their beliefs.

Why would any thinking person need to waste their time opening it,, really!!

Yeah, just open it and find out what you already knew anyway.

OP, you say if you opened it and there isn't a god, it would have a devasting impact on your life.

It shouldn't. Only in the sense that you have wasted years of your life worshipping someone who isn't there. It should be an enlightening discovery (which is was for me, when I opened my letter so to speak) to know that you no longer have to waste your time and emotional energy on "all of that religious stuff." You can view life from a different perspective and do good things for the sake of doing good things, not just because you want to get into heaven.

I infer the negative consequences you speak of are to do with a very religious girlfriend/wife? Just my guess...