8 We are mindful only of what is witnessed to by Holy Writ and what is otherwise well known. Christ proves His own divinity and the divine origin of His mission by miracles; He teaches the multitudes heavenly doctrine by word of mouth; and He absolutely commands that the assent of faith should be given to His teaching, promising eternal rewards to those who believe and eternal punishment to those who do not. "If I do not the works of my Father, believe Me not" ( John x., 37). "If I had not done among them the works than no other man had done, they would not have sin" ( Ibid . xv., 24). "But if I do (the works) though you will not believe Me, believe the works" ( Ibid . x., 38). Whatsoever He commands, He commands by the same authority. He requires the assent of the mind to all truths without exception. It was thus the duty of all who heard Jesus Christ, if they wished for eternal salvation, not merely to accept His doctrine as a whole, but to assent with their entire mind to all and every point of it, since it is unlawful to withhold faith from God even in regard to one single point. When about to ascend into heaven He sends His Apostles in virtue of the same power by which He had been sent from the Father; and he charges them to spread abroad and propagate His teaching. "All power is given to Me in Heaven and in earth. Going therefore teach all nations....teaching them to observe all things whatsoever I have commanded you" ( Matt . xxviii., 18-19-20). So that those obeying the Apostles might be saved, and those disobeying should perish. "He that believeth and is baptized shall be saved, but he that believed not shall be condemned" ( Mark xvi., 16). But since it is obviously most in harmony with God's providence that no one should have confided to him a great and important mission unless he were furnished with the means of properly carrying it out, for this reason Christ promised that He would send the Spirit of Truth to His Disciples to remain with them for ever. "But if I go I will send Him (the Paraclete) to you....But when He, the Spirit of Truth is come, He will teach you all truth" ( John xvi., 7-13). "And I will ask the Father, and He shall give you another Paraclete, that he may abide with you for ever, the Spirit of 'Truth" ( Ibid . xiv., 16-17). "He shall give testimony of Me, and you shall give testimony" ( Ibid . xv., 26-27). Hence He commands that the teaching of the Apostles should be religiously accepted and piously kept as if it were His own - "He who hears you hears Me, he who despises you despises Me" ( Luke x., 16). Wherefore the Apostles are ambassadors of Christ as He is the ambassador of the Father. "As the Father sent Me so also I send you" ( John xx., 21). Hence as the Apostles and Disciples were bound to obey Christ, so also those whom the Apostles taught were, by God's command, bound to obey them. And, therefore, it was no more allowable to repudiate one iota of the Apostles' teaching than it was to reject any point of the doctrine of Christ Himself. Truly the voice of the Apostles, when the Holy Ghost had come down upon them, resounded throughout the world. Wherever they went they proclaimed themselves the ambassadors of Christ Himself. "By whom (Jesus Christ) we have received grace and Apostleship for obedience to the faith in all nations for His name" ( Rom . i., 5). And God makes known their divine mission by numerous miracles. "But they going forth preached everywhere: the Lord working withal, and confirming the word with signs that followed" ( Mark xvi., 20). But what is this word? That which comprehends all things, that which they had learnt from their Master; because they openly and publicly declare that they cannot help speaking of what they had seen and heard. But, as we have already said, the Apostolic mission was not destined to die with the Apostles themselves, or to come to an end in the course of time, since it was intended for the people at large and instituted for the salvation of the human race. For Christ commanded His Apostles to preach the "Gospel to every creature, to carry His name to nations and kings, and to be witnesses to him to the ends of the earth." He further promised to assist them in the fulfilment of their high mission, and that, not for a few years or centuries only, but for all time - "even to the consummation of the world." Upon which St. Jerome says: "He who promises to remain with His Disciples to the end of the world declares that they will be for ever victorious, and that He will never depart from those who believe in Him" (In Matt ., lib. iv., cap. 28, v. 20). But how could all this be realized in the Apostles alone, placed as they were under the universal law of dissolution by death? It was consequently provided by God that the Magisterium instituted by Jesus Christ should not end with the life of the Apostles, but that it should be perpetuated. We see it in truth propagated, and, as it were, delivered from hand to hand. For the Apostles consecrated bishops, and each one appointed those who were to succeed them immediately "in the ministry of the word." Nay more: they likewise required their successors to choose fitting men, to endow them with like authority, and to confide to them the office and mission of teaching. "Thou, therefore, my son, be strong in the grace which is in Christ Jesus: and the things which thou bast heard of me by many witnesses, the same command to faithful men, who shall be fit to teach others also" (2 Tim . ii., I-2). Wherefore, as Christ was sent by God and the Apostles by Christ, so the Bishops and those who succeeded them were sent by the Apostles. "The Apostles were appointed by Christ to preach the Gospel to us. Jesus Christ was sent by God. Christ is therefore from God, and the Apostles from Christ, and both according to the will of God....Preaching therefore the word through the countries and cities, when they had proved in the Spirit the first-fruits of their teaching they appointed bishops and deacons for the faithful....They appointed them and then ordained them, so that when they themselves had passed away other tried men should carry on their ministry" (S. Clemens Rom. Epist. I ad Corinth . capp. 42, 44). On the one hand, therefore, it is necessary that the mission of teaching whatever Christ had taught should remain perpetual and immutable, and on the other that the duty of accepting and professing all their doctrine should likewise be perpetual and immutable. "Our Lord Jesus Christ, when in His Gospel He testifies that those who not are with Him are His enemies, does not designate any special form of heresy, but declares that all heretics who are not with Him and do not gather with Him, scatter His flock and are His adversaries: He that is not with Me is against Me, and he that gathereth not with Me scattereth" (S. Cyprianus, Ep. lxix., ad Magnum , n. I).
Source: Satis Cognitum (Vatican.va)