I know I am going to hate myself for asking this … where exactly in the Bible is it written that only the RCC and its followers may speak for God in all matters period … just as if God spoke it Himself?
First, the CC certainly doesn't speak for God in all matters (although in the final analysis all matters have to do with God)!
Only the One, Holy, Catholic and Apostolic Church was given Christ's infallible authority to preach, teach and speak in matters of the Christian Faith and morals.
As far as the individual lay members---when we say some doctrine or meaning of Scripture is true, we only repeat what the CC's official teachers (The Magisterium, the Pope and the bishops) have taught.
Christ chose His Twelve Apostles and established only one Church with St.Peter as its leader. He taught them the Faith and the way to practice that Faith. Just before He ascended into Heaven, He gave the Apostles His authority to teach and preach all nations and told them He would be with them until the end of the world. He also said He would send the Holy Spirit to teach and guide them.
The Bible says the Church is the bride of Christ. Eph. 5:23-32. Jesus can have but one spouse and that one spouse is the Catholic Church. His Church also teaches and practices one Faith which was taught and practiced first by the Apostles. St.Jude 3. This is the unity of belief to which Christ calls us. Phil. 1:27; 2:2.
The CC is the Mystical Body of Christ.....a divine body, an organic and spiritual entity. The true faithful make up the members of body while Christ reigns in Heaven as the Eternal Priest and Head.
The Bible passages that bear this out are:
Col. 1:18, "Again, He is the head of His body, the Church."
"Now you are the body of Christ, member for member." 1Cor. 12:27.
Christ's one Chruch must be a unified body. Not segmented as all the Protestant churches that began after Martin Luther in 1517.
The Bible passages are:
"There shall be one fold and one shepherd" St.John 10:16.
"And the glory that thou, Father, has given Me, I have given them, that they may be one, even as we are one." 17:22.
"You were called in one body, ...one Spirit...one hope...one Lord...one faith...one baptism." Eph. 4:4-6.
Again, Jesus chose the apostles to be the earthly leaders of His Church. First, He gave Peter special authority among the apostles. St.John 21:15-17 and signified this by changing his name from Simon to Kepha (Aramaic), Petros (Greek) and Peter (English), which means 'rock' 1:42. Christ said Peter was to be the rock on which He would build His Church.
St. Matt. 16:18-19
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.
Explanation of those things underlined are as follows:
 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, St. 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 fulness of ecclesiastical power, signified by the keys of the kingdom of heaven.
 Upon this rock: The words of Christ to Peter, spoken in the 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, St. Matt. 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 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.
Note in verse 19, Christ gave St.Peter "the keys to the kingdom" and the power and authority "to bind and loose".
The Bible often calls the Church "the kingdom"...and Peter was given the keys to Christ's Church, His kingdom. The keys are symbols of Christ's authority given to Peter to rule the Church in His absence. And He promised Peter's decisions would be binding in Heaven.
In St.Matt. 18:18, Christ gave the other apostles the same power of "binding and loosing".
SHortly before He acsended into Heaven Christ directed His teaching and preaching authority and mission to the Apostles.
" All power in Heaven and earth has been given to Me, go therefore teach 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."
The Bible teaches the teaching and preaching authority of Christ's one Church would not, could not ever err beacsue fallible teachers would not be doing the teaching.
Christ pledged divine protection to that teaching authority...
"These things I have spoken to you while yet dwelling with you. But the Advocate, the Holy Spirit,whom the Father will send in My name, He will teach you all things, and bring to your mind whatever I have said to you...when the Advocate has come, whom I will send you from the Father, He will bear witness concerning Me, becasue from the beginning you are with Me." St.John 14:25-26, 15:26-27.
In other words, the infallible Holy Spirit, the infallible Christ, would be doing the actual teaching, speaking through the human teaching authority of His Church. Christ made this quite clear when He said,
"He who hears you hears Me." St.Luke 10:16.
The human teaching authority is of His Church is the Magisterium which is the pope and bishops, the successors of the Apostles.
That's why we call the Church, Apostolic.
Christ appointed the Apostles to be the first leaders and their successors were to be its future leaders. The Apostles were the Church's first bishops. The pope is the successor of Peter as bishop of Rome. The Church's other bishops are successors to the Apostles in general. Since the first century, there has been an unbroken line of Catholic bishops faithfully handing on what the Apostles taught the first Christians in Scripture and oral tradition. 2Tim. 2:2.