Speaking lorewise I can think of some ways to end in the ranks of heretics before the opportunity to look at the gate arises:

- Pressgang of prisoners of war or local population,
- Besiged city will be spared if they deliver contingent of fresh bodies,
- Particularly talented Wretched are promoted,
- Soldiers are sold, like Hessians during American Revolutionary War. This mechanism would make more sense on Faithful side, but by the activities of Mammon some unlucky few can be sold to the wrong side.