6 We were created by God, the Creator of the universe, in order that we might know Him and serve Him; our Author therefore has a perfect right to our service. God might, indeed, have prescribed for man's government only the natural law, which, in His creation, He imprinted on his soul, and have regulated the progress of that same law by His ordinary providence; but He preferred rather to impose precepts, which we were to obey, and in the course of time, namely from the beginnings of the human race until the coming and preaching of Jesus Christ, He Himself taught man the duties which a rational creature owes to its Creator: "God, who at sundry times and in divers manners, spoke in times past to the fathers by the prophets, last of all, in these days, hath spoken to us by his Son." From which it follows that there can be no true religion other than that which is founded on the revealed word of God: which revelation, begun from the beginning and continued under the Old Law, Christ Jesus Himself under the New Law perfected. Now, if God has spoken (and it is historically certain that He has truly spoken), all must see that it is man's duty to believe absolutely God's revelation and to obey implicitly His commands; that we might rightly do both, for the glory of God and our own salvation, the Only-begotten Son of God founded His Church on earth. Further, We believe that those who call themselves Christians can do no other than believe that a Church, and that Church one, was established by Christ; but if it is further inquired of what nature according to the will of its Author it must be, then all do not agree. A good number of them, for example, deny that the Church of Christ must be visible and apparent, at least to such a degree that it appears as one body of faithful, agreeing in one and the same doctrine under one teaching authority and government; but, on the contrary, they understand a visible Church as nothing else than a Federation, composed of various communities of Christians, even though they adhere to different doctrines, which may even be incompatible one with another. Instead, Christ our Lord instituted His Church as a perfect society, external of its nature and perceptible to the senses, which should carry on in the future the work of the salvation of the human race, under the leadership of one head, with an authority teaching by word of mouth, and by the ministry of the sacraments, the founts of heavenly grace; for which reason He attested by comparison the similarity of the Church to a kingdom, to a house, to a sheepfold, and to a flock. This Church, after being so wonderfully instituted, could not, on the removal by death of its Founder and of the Apostles who were the pioneers in propagating it, be entirely extinguished and cease to be, for to it was given the commandment to lead all men, without distinction of time or place, to eternal salvation: "Going therefore, teach ye all nations." In the continual carrying out of this task, will any element of strength and efficiency be wanting to the Church, when Christ Himself is perpetually present to it, according to His solemn promise: "Behold I am with you all days, even to the consummation of the world?" It follows then that the Church of Christ not only exists to-day and always, but is also exactly the same as it was in the time of the Apostles, unless we were to say, which God forbid, either that Christ our Lord could not effect His purpose, or that He erred when He asserted that the gates of hell should never prevail against it.
Source: Mortalium Animos (Vatican.va)