Scripture needs an authoritative interpreter
Acts 8:30-31;2 Peter 1:20-21  Understanding this first, that no prophecy of scripture is made by private interpretation.
No prophecy of scripture is made by private interpretation: This shews plainly that the scriptures are not to be expounded by any one's private judgment or private spirit, because every part of the holy scriptures were written by men inspired by the Holy Ghost, and declared as such by the Church; therefore they are not to be interpreted but by the Spirit of God, which he hath left, and promised to remain with his Church to guide her in all truth to the end of the world. Some may tell us, that many of our divines interpret the scriptures: they may do so, but they do it always with a submission to the judgment of the Church, and not otherwise.
 For prophecy came not by the will of man at any time: but the holy men of God spoke, inspired by the Holy Ghost.
2 Peter 3:15-16  And account the long suffering of our Lord, salvation; as also our most dear brother Paul, according to the wisdom given him, hath written to you:  As also in all his epistles, speaking in them of these things; in which are certain things hard to be understood, which the unlearned and unstable wrest, as they do also the other scriptures, to their own destruction.
Matthew 16:13-20 - And Jesus came into the quarters of Caesarea Philippi: and he asked his disciples, saying: Whom do men say that the Son of man is?  But they said: Some John the Baptist, and other some Elias, and others Jeremias, or one of the prophets.  Jesus saith to them: But whom do you say that I am?  Simon Peter answered and said: Thou art Christ, the Son of the living God.  And Jesus answering, said to him: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven.  And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it.  And I will give to thee the keys of the kingdom of heaven. And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose upon earth, it shall be loosed also in heaven.  Then he commanded his disciples, that they should tell no one that he was Jesus the Christ.
Christ left a Church with divine authority
to teach in His name
Matthew 18:18 Amen I say to you, whatsoever you shall bind upon earth, shall be bound also in heaven; and whatsoever you shall loose upon earth, shall be loosed also in heaven.;
Luke 10:16 He that heareth you, heareth me; and he that despiseth you, despiseth me; and he that despiseth me, despiseth him that sent me.).
Thou art Peter: As St. Peter, by divine revelation, here made a solemn profession of his faith of the divinity of Christ; so in recompense of this faith and profession, our Lord here declares to him the dignity to which he is pleased to raise him: viz., that he to whom he had already given the name of Peter, signifying a rock, John 1:42, should be a rock indeed, of invincible strength, for the support of the building of the church; in which building he should be, next to Christ himself, the chief foundation stone, in quality of chief pastor, ruler, and governor; and should have accordingly all fullness of ecclesiastical power, signified by the keys of the kingdom of heaven.
Upon this rock: The words of Christ to Peter, spoken in the vulgar language of the Jews which our Lord made use of, were the same as if he had said in English, Thou art a Rock, and upon this rock I will build my church. So that, by the plain course of the words, Peter is here declared to be the rock, upon which the church was to be built: Christ himself being both the principal foundation and founder of the same. Where also note, that Christ, by building his house, that is, his church, upon a rock, has thereby secured it against all storms and floods, like the wise builder, Matthew. 7: 24-25.
The gates of hell: That is, the powers of darkness, and whatever Satan can do, either by himself, or his agents. For as the church is here likened to a house, or fortress, built on a rock; so the adverse powers are likened to a contrary house or fortress, the gates of which, that is, the whole strength, and all the efforts it can make, will never be able to prevail over the city or church of Christ. By this promise we are fully assured, that neither idolatry, heresy, nor any pernicious error whatsoever shall at any time prevail over the church of Christ. Loose upon earth: The loosing the bands of temporal punishments due to sins, is called an indulgence; the power of which is here granted.
The Church will last until the end of time,
and the Holy Spirit protects the
Church's teaching from corruption
Matthew 28:18-20  And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth.  Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.  Teaching them to observe all things whatsoever I have commanded you: and behold I am with you all days, even to the consummation of the world.
All power: See here the warrant and commission of the apostles and their successors, the bishops and pastors of Christ's church. He received from his Father all power in heaven and in earth: and in virtue of this power, he sends them (even as his Father sent him, John 20:21) to teach and disciple, not one, but all nations; and instruct them in all truths: and that he may assist them effectually in the execution of this commission, he promises to be with them, not for three or four hundred years only, but all days, even to the consummation of the world. How then could the Catholic Church ever go astray; having always with her pastors, as is here promised, Christ himself, who is the way, the truth, and the life.
John 14:16 And I will ask the Father, and he shall give you another Paraclete, that he may abide with you forever). Paraclete: That is, a comforter: or also an advocate; inasmuch as by inspiring prayer, he prays, as it were, in us, and pleads for us. For ever: Hence it is evident that this Spirit of Truth was not only promised to the persons of the apostles, but also to their successors through all generations.
The Church and not the Bible alone is the
"pillar and bulwark of the truth"
1 Timothy 3:15 But if I tarry long, that thou mayest know how thou oughtest to behave thyself in the house of God, which is the church of the living God, the pillar and ground of the truth.
John 1:1, 14 In the beginning was the Word, and the Word was with God, and the Word was God, And the Word was made flesh, and dwelt among us, (and we saw his glory, the glory as it were of the only begotten of the Father,) full of grace and truth.), -The divinity and incarnation of . John bears witness of him. He begins to call his disciples
- And in 1 Thessalonian 2:13 "Therefore, we also give thanks to God without ceasing: because, that when you had received of us the word of the hearing of God, you received it not as the word of men, but (as it is indeed) the word of God, who worketh in you that have believed.", St. Paul's first epistle, he refers to "the Word of God which you heard from us." There St. Paul is clearly referring to oral apostolic teaching: tradition.
- In fact, what this and similar texts warn against is the addition of human wisdom to the Word of God. Scripture does not claim that God can only hand down revelation in written form. If God chose to reveal Himself further after the writing of Deuteronomy, and He did, or chooses to reveal Himself through some means other than writing - and He did, notably in the Incarnation of Jesus Christ - then the prohibition of Deuteronomy 4:2 "You shall not add to the word that I speak to you, neither shall you take away from it: keep the commandments of the Lord your God which I command you" does not apply.