It's possible to form the HRE (or at least, pass loads of reforms) by using the Heresy event, or losing a war to the russians, to change religion. Then, you convert loads of HRE minors to your religion, to get authority, pass reforms, passing up until the last two reforms, change back to catholicism, for the huge relations boost to everyone, convert those minors back, and pass the last two reforms.
How do you get anyone to vote for you if almost all the electors are heretics?