What we have to bear in mind is that things were and are never clear cut. Lots of medieval nobles genuinely believed that if they didn't cancel out their sins they would burn for all eternity. Of course, since sinning was so popular the church created a loophole whereby nobles could buy salvation through donations to the church as long as they at least kept their sinning private. But even most of those who did commit adultery and murder people still believed in the religious doctrine and generally went out of their way to appear pious - not to mention condemning anyone who publicly flouted religious customs.
However, that didn't me that nobles didn't see the power of religion and didn't use it. After all, many of the great events in european history were motivated by a combination of piety and pragmatism. Such as the crusades, for example. Go on one this and you have the opportunity to get rich by taking other people's lands - but at the same time it's the pious thing to do and it will redeem your soul.
And, to be honest, I think that's how most medieval nobles would have behaved. They would have been believers but they'd also have balanced the pragmatic considerations of any situation as well.
So I don't really see anything wrong with characters using religion for political ends - but just bluntly using it as a tool without any figleafs or having a meaningless religion (like the Path of Chivalry in Oritolon) is the kind of thing which I do think should be seriously frowned upon.
