You can’t. Not within the framework that it currently requires.
Theocracy is a fatally flawed concept, beginning from the point where because it’s theocracy, it can’t be flawed. If everything’s hinging on ‘god’s will’, then as soon as mistakes get made, you’ve got evidence that god’s not actually behind it. And mistakes get made, constantly. Governance requires mistakes, because governance requires trying things, and some portion of them will inevitably fail.
So if the person in charge is screwing up, you replace them… but if god wanted them in charge, then… what? Everyone who said ‘It’s god’s will!’ has to publicly admit they didn’t have the first clue WTF ‘god’ wanted? God lied? Or was god just being a mean-spirited bitch?
None of those options will keep a theocracy in power. And any form of government that vests power in the leadership, without making that leadership responsible to those who are led, is all about maintaining power. Even in the State, where the upper levels of the Megacorps are almost as hereditarily-stratified as the Empire, the top levels are answerable to lower levels through the corporate structure and the nature of corporate shareholders.
Theocracy doesn’t allow for that. Theocracy hinges directly on refuting that bottom-up accountability. It is why theocracy will always degrade quickly into either a dictatorship, a hereditary aristocracy, or (as in the case of the Amarr Empire), a quasi-feudal nobility of hereditary oligarchs overseen by a supreme executive chosen from among their ranks. And whichever form arises, it will maintain the trappings of the faith, even as that faith serves only to reinforce the power structure from which the ‘church leaders’ benefit.
The most interesting wrinkle the Empire’s introduced, really, is Shathol’syn. That, at least, yokes the Heirs to the current supreme executive by effectively saying ‘any one of you assholes has me killed, there’s a 5 in 6 chance you’ll die’. It lends a bit of additional stability and rigidity to an already ossified structure.