12 Now, this authority, perfect in itself, and plainly meant to be unfettered, so long assailed by a philosophy that truckles to the State, the Church, has never ceased to claim for herself and openly to exercise. The Apostles themselves were the first to uphold it, when, being forbidden by the rulers of the synagogue to preach the Gospel, they courageously answered: "We must obey God rather than men."(16) This same authority the holy Fathers of the Church were always careful to maintain by weighty arguments, according as occasion arose, and the Roman Pontiffs have never shrunk from defending it with unbending constancy. Nay, more, princes and all invested with power to rule have themselves approved it, in theory alike and in practice. It cannot be called in question that in the making of treaties, in the transaction of business matters, in the sending and receiving ambassadors, and in the interchange of other kinds of official dealings they have been wont to treat with the Church as with a supreme and legitimate power. And, assuredly, all ought to hold that it was not without a singular disposition of God's providence that this power of the Church was provided with a civil sovereignty as the surest safeguard of her independence.
Source: Immortale Dei (Vatican.va)