On page 18 of the playtest errata the redemption paladin can take the "Weight of Guilt" feat in order to allow "Glimpse of Redemption" to stupify instead of enfeeble the enemy. Page 324 of the rulebook states that if you are stupified and want to cast a spell you must succeed on a spell roll against the DC of the effect that gave you the stupified condition. So if someone gets stupified by "Weight of Guilt" what is the DC for the spell roll?
Looks like a flub on their part, but I'd go with
Your Difficulty Class
Often, you roll dice against a Difficulty Class determined by the GM. But when a creature or situation is testing your character’s ability, it attempts a check against a Difficulty Class based on the most relevant of your character’s statistics. The DC for any statistic is 10 plus all the same modifiers you’d add to a d20 roll using that statistic.
The most relevant stat feels like it should definitely be Cha, but I don't think I could justify that with rules text.
The stat in question would be the Paladin class's primary statistic (your class DC), as covered by the general rules that I don't have access to at the moment to quote.