What ability score for archetype derived Focus Spells? (Invoke the Crimson Oath)

[cavernshark]

What ability score does a non-caster Knight Reclaimant use to cast Invoke the Crimson Oath?

The Knight Reclaimant archetype has access to a feat called Invoke the Crimson Oath which gives them a focus pool and a focus spell of the same name: invoke the crimson oath.

Invoke the Crimson Oath wrote:

Feat 8; Archetype

Source Lost Omens Character Guide pg. 95
Archetype Knight Reclaimant
Prerequisites Knight Reclaimant Dedication
You can invoke the Crimson Oath to unleash blasts of ruby energy from your weapon. You gain the invoke the Crimson Oath focus spell. If you don’t already have one, you gain a focus pool of 1 Focus Point, which you can regain using the Refocus activity to recite the Crimson Oath and meditate on its teachings; if you already have a focus pool, increase the number of Focus Points in your focus pool by 1. Your knight reclaimant focus spells are divine spells; when you gain this feat, you become trained in divine spell attacks and spell DCs.

Focus spells are usually derived from base classes which have key ability scores (e.g. Champion or Monk). It is entirely possible, however, for a fighter, rogue or some other non-spellcaster to become a Lastwall Sentry and then a Knight Reclaimant. Invoke the Crimson Oath could be the first and only focus power acquired by the character. The feat accounts and gives training in the divine spell attacks and spell DCs, but doesn't say which ability modifier to use? Is it assumed Wisdom because it's divine, is it the key ability score of the class for class DCs (str or dex for fighters?), or something else?