The propagation of the Agmon faith in the Olympian city has been going on for some time.
Alton was very sneaky with the way he chose his targets. His primary focus was on the people that are not affiliated with any academy. They are the most desperate ones right now. Even if they managed to snag a mission or two and earned the weekly payment for the hub, they are being targeted by the academy forces.
Since they don't have any backing, they don't have any protection from the city guards as well.
Even the people that they were previously serving had turned their backs on them and left these people to survive on their own. This made them vulnerable and open to this new path. They were easily persuaded and no one actually missed them in the academy realm, when they went to the Church of Agmon.
Alton even went as far as showcasing his payment status in the Hub's interface.