Because we know that its inspired by the Holy Spirit, is infallible, and has not contradictions or errors. Now, you may need to tell us which of the following positions of the RCC are the infallible ones and inspired by the Holy Spirit.
Pope Boniface VIII, Unam Sanctam: We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff. Pope Boniface VIII, Unam Sanctam (Promulgated November 18, 1302) http://www.fordham.edu/halsall/source/b8-unam.html
St. Thomas Aquinas: It is also shown that to be subject to the Roman Pontiff is necessary for salvation. St. Thomas Aquinas, Against the Errors of the Greeks, Pt. 2, ch. 36 http://dhspriory.org/thomas/ContraErrGraecorum.htm#b38
that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church. Pope Eugene IV, Cantate Domino, Bull promulgated on February 4, 1441 (Florentine style), proclaimed ex cathedra (infallible).
The Holy Catholic Church teaches that God cannot be adored except within her fold; she affirms that all those who are separated from her will not be saved. (Pope St. Gregory the Great, Moralia, XIV:5)
Neither the true Faith nor eternal salvation is to be found outside the Holy Catholic Church. It is a SIN to believe that there is salvation outside the Catholic Church. (Ven. Pope Pius IX, Singulari Quidem, March 17, 1856; cf. also OUR GLORIOUS POPES, Slaves of the Immaculate Heart of Mary, Cambridge, MA: 1955, p.168)
Then a change,
RCC: 846 How are we to understand this affirmation, often repeated by the Church Fathers?[335] Re-formulated positively, it means that all salvation comes from Christ the Head through the Church which is his Body: Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it.[336]
..there are many who honor Sacred Scripture, taking it as a norm of belief and a pattern of life, and who show a sincere zeal. They lovingly believe in God the Father Almighty and in Christ, the Son of God and Saviour. (Cf. Jn. 16:13) They are consecrated by baptism, in which they are united with Christ. They also recognize and accept other sacraments within their own Churches or ecclesiastical [Protestant] communities
They also share with us in prayer and other spiritual benefits. Likewise we can say that in some real way they are joined with us in the Holy Spirit, for to them too He gives His gifts and graces whereby He is operative among them with His sanctifying power. Some indeed He has strengthened to the extent of the shedding of their blood. LUMEN GENTIUM: 16.
those who are baptized in these communities are, by Baptism, incorporated in Christ and thus are in a certain communion, albeit imperfect, with the Church. All who have been justified by Faith in Baptism are incorporated into Christ: they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church. http://www.vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_20000806_dominus-iesus_en.html
RCC: 847 This affirmation is not aimed at those who, through no fault of their own, do not know Christ and his Church: Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience those too may achieve eternal salvation.[337]
We can rely on the consistency of scripture. How about your RCC?
Pope Boniface VIII, Unam Sanctam (Promulgated November 18, 1302) http://www.fordham.edu/halsall/source/b8-unam.html
NOW we know where the MORMONs got it from!!!!
They succeeded in killing Joseph, but he had finished his work.He was a servant of God, and gave us the Book of Mormon.He said the Bible was right in the main, but, through the translators and others, many precious portions were suppressed, and several other portions were wrongly translated; and now his testimony is in force, for he has sealed it with his blood.As I have frequently told them, no man in this dispensation will enter the courts of heaven, without the approbation of the Prophet Joseph Smith, Jun.Who has made this so?Have I?Have this people?Have the world?No; but the Lord Jehovah has decreed it.If I ever pass into the heavenly courts, it will be by the consent of the Prophet Joseph.If you ever pass through the gates into the Holy City, you will do so upon his certificate that you are worthy to pass.Can you pass without his inspection?No; neither can any person in this dispensation, which is the dispensation of the fulness of times.In this generation, and in all the generations that are to come, everyone will have to undergo the scrutiny of this Prophet.They say that they killed Joseph, and they will yet come with their hats under their arms and bend to him; but what good will it do them, unless they repent?They can come in a certain way and find favor, but will they?
--JOURNAL OF DISCOURSES, vol. 8, p. 224
Thank you for documenting that the Bible doesn’t say affirmatively that it is the ONLY source. The question was for the prots to prove that it does. Have you decided to become Catholic?