This is a very long letter! By the time it ends he has forgotten the beginning, several times over.
Luckily, the mystery voice (Willow on behalf of Ash?) is willing to repeat itself. After enough repetitions, he thinks he has memorized most of it, and understood almost nothing.
Some parts are at least easy to not understand. How can something be "beyond Creation", isn't Creation the word for everything that exists? How could Golarion be destroyed, and if it was destroyed how could it be remade, all in a few minutes? Who is Rovagug? Where is Aelsef and what's special about Abadar's temple there? Why is the god of drunkedness certifying anything alongside Abadar? If Asmodeus objected to something in Hell, why did He destroy Golarion?
Other parts are more... troubling.
That door leads to where he was, that is, the temple of Abadar in his village. Did they add a door to the temple, and build this... weird room next to it? But there isn't room to build it next to the temple, it would block one road or another. Did the priest actually agree to any of this? What gives them the right to give him a building located on a public road?
They say they've given him - this room, and food, and magic spells? How can he possibly ever pay them for that? They say it's a gift made 'in apology', but the apology is for saving his life (?) and that's not how apologies work (??) He can't accept a gift like this! It's the farthest possible thing from the fair trades Abadar wants people to make! If he accepts it, he'll probably never make it to Axis!
Speaking of which, it's very scary that he's being directed to a non-Lawful visitors part of Axis. He's very very sure he wants the Lawful residents part instead. Just to be sure he doesn't accept this horribly unfair deal, he won't eat any of the food or use any of the magic.
That leaves him with only one option: to ask the priest! Which is obviously the right thing to do, and the letter also says so, he really shouldn't have spent so long trying to puzzle it out himself.
He opens the door.