11 When the law, by the suppression of the Budget of Public Worship, exonerates the State from the obligation of providing for the expenses of worship, it violates an engagement contracted in a diplomatic convention, and at the same time commits a great injustice. On this point there cannot be the slightest doubt, for the documents of history offer the clearest confirmation of it. When the French Government assumed in the Concordat the obligation of supplying the clergy with a revenue sufficient for their decent subsistence and for the requirements of public worship, the concession was not a merely gratuitous one - it was an obligation assumed by the State to make restitution, at least in part, to the Church whose property had been confiscated during the first Revolution. On the other hand when the Roman Pontiff in this same Concordat bound himself and his successors, for the sake of peace, not to disturb the possessors of property thus taken from the Church, he did so only on one condition: that the French Government should bind itself in perpetuity to endow the clergy suitably and to provide for the expenses of divine worship.
Source: Vehementer Nos (Vatican.va)