Alright people, hear me out on this one.
Once upon a time the Elven 'gods' roamed Thedas, as guardians of the world. Think of them as like a Superman Police Force. All they do is that. But eventually, 7 of them become power hungry. "Why are we just guardians? Can't we rule this world?". The remaining two, also known as Fen'Harel and Mythal, disagree. Fighting breaks out between the two groups.
Those 7 are the Old Gods. Maybe they liked roaming the lands in their dragon-forms or something. But eventually, Fen'Harel and Mythal manage to overcome them, and lock them up, deep beneath the earth. Crisis averted, you should think. But no. One of the imprisoned 'gods', to us known as Dumat, hates sitting in his 2 by 2 cell so he... I don't know, finds a way to get some magisters to break into the Fade and enter the Golden City. Maybe he thought the Dread Wolf ("I bet Fen is sitting there on a throne, the backstabbing dog!") had made that into his throneroom or something. We don't have all that much to go on here. Corypheus's ramblings don't help either. Maybe he just wanted vengeance (he could be Elgar'nan).
And then a Blight happens. Fen'Harel and Mythal are like: "Sh*t." and help that crisis be averted. Then they get together.
"It's not working like this," Mythal says. "We have to come up with a plan."
"Our brothers and sisters still have influence," responds Fen'Harel. "What do you want to do?"
"I propose we bring forth a new faith. One god, a creator... a 'Maker', if you will. We spread this faith to all the corners of Thedas and let the people rise behind it. And then we back down. The hold of our brothers and sisters over this world will be broken, and mankind can start to rule itself."
"Excellent idea," says Fen'Harel. "How do you want to go about doing that?"
"Easy. I rise to power as a southerner, rallying an army behind me. Then I march on the oppressors, the Vints."
"That could work. I'll help."
Andraste... and Shartan. We know what Shartan looks like, according to the Sacred Ashes-questline in DA:O. He was a bald elf.
So these two march on the Imperium, meanwhile spreading the faith of the Maker. And, also just like they planned, that is where Andraste is betrayed by Maferath (who is influenced by the cunning trickster Shartan), and dies. The world now has a prophet, and the Chantry will be born.
"What are you going to do after, Mythal?"
"Thinking of retreating to the wilderness. You?"
"I might roam the lands. Maybe visit the Fade a few times, live the memories of the people. I could use a vacation."
"Hopefully it will be a long time before someone tries to rule like a god again."
"Hopefully."
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Now, quick, someone point out the flaws in my theory before I turn it into fanfiction.