I read it originally, I think of it, as Honor.

But you also want to draw new, uninvolved, readers in.

While not utterly mis-representing what your story is about.

So.

I thought, The Honor of the Dead. Then, Fighting against (not "to fight with) Heaven.

How about splitting the difference?

A Sword against the Gods

Grappling with while not actually contradicting the universe you're creating, while replicating Kiril's--well, his isolation and his lingering faith and his idealism and his hopelessness, yes? And it allows you as author to be simultaneously believer and atheist...

And I think it captures Kiril and Thanike's odd position of being True Believer-Accepted Follower/Avowed Renegade--Known Traitor.....

And, after all (re my suggested title), if the gods had ever wanted Kiril NOT to raise his Mum's sword against them, They had only ever to say so.....

Butif that were the title, you'd have to rewrite the first chapter to show that Kiril and Niki were taking on, not just the legalese, the judicial apparatus, of their world in that great Renunciation:

They were defying apparently succsessfully) supernatural retribution.