It was both, as so often in similar enterprises of the age, from the Palestine to the Baltic crusades, in the Albigensian Crusade, State and Church came together in one unholy alliance.

Although it seems that the first official impetus for the Crusade came from the Catholic Church, it seems unlikely that it did so without consulting first those that would supply the military means.

For the Papal authorities, it had become necessary to react upon the rapid spreading of the Cathar dualist heresy in the Languedoc, from where it was beginning to inform similar groups all over Central Europe.

For the French Kings, Phillip Augustus and his successor Louis  VIII, it offered a unique opportunity to extend their rule over the rich Southern parts of France, that under the independent rule of the Counts of Toulouse and under the protection of the Kings of Aragon, had  economically and culturally surpassed the Northern territories of France.

And for a whole number of smaller feudal lords, Simon de Montfort being the most famous one, the Crusade against the Cathars offered the same spoils as the one in the Holy Land, only with less travel and stress.

 

This was hardly an isolated incident.  The Church eventually collaborated with Phillip IV and engineered another land grab by destroying the influence and position of the Templars 100 years later.  The Templars were accused of heresy among other deviations.

 

In the old days you made wealth "the old fashioned way."  You took it away from somebody else.