Why shouldn't you be able to declare a realm lacking followers as faithful?
Imagine just the ruler converts– I can see a religion declaring a realm faithful based on a single high-profile conversion. See "Clovis" for details.
Sure, the ruler being faithful is a possible test. I said there were no easy metric, not that there were no metric at all.
The example of the conversion of Clovis is a very good example where your mechanics does work, but your mechanic as no restriction at all to limit it to those cases.
In your example, the Italian Pope could declare that heathen vikings are faithful and send priests to incite northern french catholic peasants to abandon the French King, hoping to weaken him and take the territories of Southern France. That would be a clever and legitimate power play on the part of the Italians, but it just doesn't make any sense from a religious point of view.
And that would hardly be an exploit. The puppet state would gain from the priests, sure, but the main state would be gaining enormous power and sway over the other state– which would incentivize the smaller state to get some priests and elders so it can have a little control of its own destiny.
Ok, maybe so, but it's indirect and contrived. If you want to convert your puppet state, why don't you preach in it?
