@OoohTheStatsDontLie - I think you've really hit the nail on the head with your comments.
Yes, calling it "tedious" isn't the worst thing ever - but bearing in mind it's to someone who's incredibly rude and hostile about the OP, stirring up even more bad feeling towards their family unit would be upsetting.
And I also don't get the real vitriol towards you about reading his messages. You've owned it, hands up, that it wasn't the loveliest thing to do but I"ve read thread after thread after thread recently where MN has URGED the OP to look at their partner's phone! She did it, she's not proud of it, and she knows she shouldn't have. Enough of the preaching at her!
I also think I'd be extremely concerned that he wants to divorce you because you looked at that message. Huge overreaction. What is he hiding? Sounds like there is something he has a guilty conscience over....not suggesting he's cheating, but possibly "worse" messages to his friend....
I don't look at my partner's phone for this reason. I know he'd be blowing off steam to friends if we're having a hard time, and I know I would blow it out of proportion and take it personally. But if I did look and confess....he wouldn't be impressed with me....but I guarantee you he wouldn't instantly threaten to walk out. Nor would I if he snooped through my messages.
My angry self feels like saying that he can fuck off if he's going to treat you like that, let him piss off back to his pot-head mate and you'll be better off without him! But of course, that's not really the adult response and you should really talk to him when you've both calmed down. Yes, you need to apologise, but he owes you a big apology - and explanation - too.