I think that tends to vary by region and state.
Clearly the Medici's were not a very religious bunch, and they installed a couple of Popes (there is one great letter in which instructions are given on how to make sure the Pope pays his bills). The nature of any given state probably influences just how cynical the lords are likely to be. Nobility in England or Italy were in constant danger and thus the "fiddly bit" in religion were glossed over in favour of practicality. In later Spain, there were not the same concerns, so leaders would naturally become less cynical and "stabby".
Wonkspeak: True faith is probably like absolute morality and can be expressed as a luxury good in the personal consumption matrix.
