Friends, I would know the answer to the question:
A member of my group is playing a cleric with the archetype Theologian, and when reached the 10th level, he chose disintegrate to apply a metamagic feat, but would only be available at level 13. He did something illegal?
Domain secret doesn't specify that it must be a spell the cleric has the ability to cast, so I suppose by RAW he is correct.
I can't figure out which of the available metamagic feat would be nice on a disintegrate, save bouncing maybe :/
Multiple abilities that choose a particular spell don't require that it be a spell you can cast yet.
I don't see anything against the rules with choosing a higher-level spell than you can currently cast for your Domain Secret.
Ok, Thanks for the answers...