I am playing the Old Gods DLC, started in the north as a Pomeranian duke, formed the Wendish Empire and have now reconquered much of the Byzantine Empire, whose title I have usurped. I stayed as a (reformed) pagan. I seem to remember that the Byzantine Emperor has two unique decisions available: mend the schism and restore the Roman Empire. Being a pagan, it makes perfect sense that mend the schism is not available to me. However, I would have expected the Restore the Roman Empire decisions to be available. To clarify, I am not saying the decision is greyed out, it is not listed as a possible decision at all.
Are there some conditions I must fulfill before the decision will appear as an option (even if greyed out)? It would be great to know the exact list of conditions.