13 Of the rights of the Church that it is Our duty everywhere and always to maintain and defend against all injustice, the first is certainly that of enjoying the full freedom of action she may need in working for the salvation of souls. This is a divine liberty, having as its author the only Son of God, Who by shedding of blood, gave birth to the Church Who established it until the end of time, and chose Himself to be its Head. This liberty is so essential to the Church, a perfect and divine institution, that they who attack this liberty at the same time offend against God and their duty. For as We have elsewhere more than once shown, God established His Church to protect and distribute what is of supreme good to souls, by their nature superior to all others, and to bring men, by means of faith and grace, to a new life in Jesus Christ, a life that ensures eternal salvation. Since the character and rights of any society are fixed by its reason for existing and by the end it aims at, in accordance with the terms of its existence, and conformably with its object, it naturally follows that the Church is a society as distinct from civil society as their reason for existence and ends are different; it follows that she is an indispensable society, for all mankind, since all are called a Christian life, and so they who refuse to enter it, or leave it are separated for ever from life eternal; and it is a society eminently independent, and above all others, because of the excellence of the heavenly and immortal blessings towards which it tends. But an essentially free institution requires, as all may see, freedom to use the means necessary for its operations. The Church therefore needs, as proper and necessary means, the power of handing down Christian doctrine, of giving the Sacraments, of exercising divine worship, of regulating and ruling all ecclesiastical discipline, with which gifts and offices God willed that His Church should be invested and strengthened, and by an admirable providence willed too that She alone should possess. To Her alone has He given in charge all He has revealed to men and established as sole interpreter, judge, and mistress, most wise and infallible, of the truth, whose precepts states as well as individuals must hear and accept. It is equally certain the He has given the Church full freedom to judge and decide as to the things that may best suit Her ends. Wherefore it is unjustly that the civil powers take offence at the freedom of the Church, since the principle of civil and religious power is one and the same, namely, God. Therefore there can be no discord between them, nor mutual obstacles nor encroachments, for God cannot be at variance with Himself, and there cannot be conflict between His works; rather there is between them a marvellous harmony of causes and effects. It is clear likewise that when the Catholic Church, obeying Her Master's will, carries far and wide her standard among nations, she does not invade the territory of the civil power, and interferes with it in no way, but, on the contrary, protects and guards those nations, just as the Christian law does not cloud the light of human reason but adds to its brilliancy by turning it aside from falsities into which human nature easily falls, or in opening to it a newer and wider intellectual horizon.
Source: Officio Sanctissimo (Vatican.va)