First, Protestants believe that many demnominations of Christianity are in fact true and pleasing to God, despite that the Bible teaches exactly to the contrary of this opinion. They believe that there is nothing wrong with denominationalism. The Bible says that the Church is not a denomination; it clearly states that there is only ONE Church.
We will be using quotations from the Douay Rheims Challoner translation (the KJV has been used previously to prove other Catholic doctrines, so there is no need to use it again here).
Jesus promised to build only One Church.
St. Matthew 16:16-18-17: “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.”
Ephesians 4:4: “One body and one Spirit; as you are called in one hope of your calling. 5 One Lord, one faith, one baptism.”
What is this One body?
Romans 12:4-5: “For as in one body we have many members, but all the members have not the same office: So we being many, are one body in Christ, and every one members one of another.”
Ephesians 1:22-23: “And he hath subjected all things under his feet, and hath made him head over all the church, Which is his body, and the fullness of him who is filled all in all.”
Colossians 1:18: “And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.”
The Church, therefore is the body of Christ, and it is ONE. The Bible condemns the very principle of denominationalism.
St. John 17:20-21: “And not for them only do I pray, but for them also who through their word shall believe in me; That they all may be one, as thou, Father, in me, and I in thee; that they also may be one in us; that the world may believe that thou hast sent me.”
1 Corinthians 1:10: “Now I beseech you, brethren, by the name of our Lord Jesus Christ, that you all speak the same thing, and that there be no schisms among you; but that you be perfect in the same mind, and in the same judgment.”
Ephesians 4:3: “Careful to keep the unity of the Spirit in the bond of peace.”
1 St. Peter 3:8: “And in fine, be ye all of one mind, having compassion one of another, being lovers of the brotherhood, merciful, modest, humble”
Ephesians 4:5: “One Lord, one faith, one baptism.”
St. John 17:20-23: “And not for them only do I pray, but for them also who through their word shall believe in me; That they all may be one, as thou, Father, in me, and I in thee; that they also may be one in us; that the world may believe that thou hast sent me. And the glory which thou hast given me, I have given to them; that they may be one, as we also are one: I in them, and thou in me; that they may be made perfect in one: and the world may know that thou hast sent me, and hast loved them, as thou hast also loved me.”
The Lord prays that His faithful be one as He and the Father are one, and that they may be made perfect in one. So called Christian denominations who differ in doctrines cannot be the true faith, since these differences are characteristic of division, not unity.
St. Matthew 15:13: “But he answering them, said: Every plant which my heavenly Father hath not planted, shall be rooted up.”
So we can clearly see that the Bible teaches that the true Christians will not be divided in their beliefs and doctrines as are the countless contradictory denominations. As the last verse from Matthew 15 above says, those who deviate from the faith brought by Jesus Christ "shall be rooted up."
So if you still believe in denominationalism, you are disobeying the Bible's clear teaching that the Church is one and is unified in her doctrine. The Bible teaches it plain as day for those who humble themselves and adopt a spirit of sacrifice and willingness to suffer for God, if it so pleases Him.
Now for the justification by faith alone and 'once saved, always saved' arguments, which also go directly against the Scriptures. Here are presented certain passages, and you can choose either to believe what you want and ignore them, or admit that they are there for a reason and that God is calling you to humble yourself and change your path so as to avoid certain destruction.
Galatians 2:16: “But knowing that man is not justified by the works of the law, but by the faith of Jesus Christ; we also believe in Christ Jesus, that we may be justified by the faith of Christ, and not by the works of the law: because by the works of the law no flesh shall be justified.”
Many people like to argue that the above verse is proof that a person is justified by faith alone. We need to do is get this false argument out of the way. If you read the whole of this chapter of St. Paul's Epistle to the Galatians, you will readily see that the dispute is not whether or not a person is justified by faith alone. It is a dispute over whether works of the Old Law (i.e. circumcision) are sufficient for justification. St. Paul goes on to conclude that if the works of the Old Law were indeed sufficient, then Christ died in vain.
Besides, later on in the same epistle, St. Paul explicitly teaches what I have just explained.
Galatians 5:6: “For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision: but faith that worketh by charity."
Faith that what? WORKETH by charity. Now that we have gotten that out of the way, what does the rest of the Bible have to say about justification and works?
St. Matthew 5:20: “For I tell you, that unless your justice abound more than that of the scribes and Pharisees, you shall not enter into the kingdom of heaven.”
St. Matthew 10:22: “And you shall be hated by all men for my name's sake: but he that shall persevere unto the end, he shall be saved.”
Romans 13:11: “And that knowing the season; that it is now the hour for us to rise from sleep. For now our salvation is nearer than when we believed.”
1 Corinthians 3:15: “If any man's work burn, he shall suffer loss; but he himself shall be saved, yet so as by fire.”
This one from St. Paul to the Corinthians is particularly interesting because to Protestants it doesn't mean anything, but Catholics recognize it as one of MANY Biblical proofs for the reality of Purgatory's existence (see chapter 11), along with St. Matthew 5:26, James 3:2, Hebrews 12:14, Apocalypse 21:27, St. Matthew 12:32, 36, 1 Peter 3:18-20; 4:6.
St. James 2:17, 20, 24, 26: “So faith also, if it have not works, is dead in itself. …But wilt thou know, O vain man, that faith without works is dead? …Do you see that by works a man is justified; and not by faith only? …For even as the body without the spirit is dead; so also faith without works is dead.”
Apocalypse (Revelation) 20:12: “And I saw the dead, great and small, standing in the presence of the throne, and the books were opened; and another book was opened, which is the book of life; and the dead were judged by those things which were written in the books, according to their works.”
Philippians 2:12: “Wherefore, my dearly beloved, (as you have always obeyed, not as in my presence only, but much more now in my absence,) with fear and trembling work out your salvation.”
Why fear and trembling if one is already saved just for believing?
St. Matthew 7:21: “Not every one that saith to me, Lord, Lord, shall enter into the kingdom of heaven: but he that doth the will of my Father who is in heaven, he shall enter into the kingdom of heaven.”
Okay so quite clearly our Saviour, Christ Himself, tells us that believing in Him is not enough. Knowing He is God is not enough unless we do the will of God the Father. How do we do that?
St. John 14:23: “Jesus answered, and said to him: If any one love me, he will keep my word, and my Father will love him, and we will come to him, and will make our abode with him.”
Romans 8:28: “And we know that to them that love God, all things work together unto good, to such as, according to his purpose, are called to be saints.”
Obedience. And yes that includes obedience to the Church established by Jesus Christ, upon His Apostles, with St. Peter as it's earthly head.
St. Matthew 16:18: “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.”
St. Matthew 18:17: “And if he will not hear them: tell the church. And if he will not hear the church, let him be to thee as the heathen and publican.”
St. Luke 10:16: “He that heareth you (His Apostles), heareth me; and he that despiseth you, despiseth me; and he that despiseth me, despiseth him that sent me.”
St. John 15:18: “If the world hate you, know ye, that it hath hated me before you.”
Ephesians 2:20-21: “Built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone: In whom all the building, being framed together, groweth up into an holy temple in the Lord.”
The word of Jesus is not only contained in the Bible. It is also contained in the traditions of the Catholic Church. Rather than get into Catholic teachings however, what I will do is prove using the Bible, that the Bible alone is not the only source of doctrine. There is no ‘Sola Scriptura’.
St. John 21:25: “But there are also many other things which Jesus did; which, if they were written every one, the world itself, I think, would not be able to contain the books that should be written.”
1 Corinthians 11:2: “Now I praise you, brethren, that in all things you are mindful of me: and keep my ordinances as I have delivered them to you.”
2 Thessalonians 2:14: “Therefore, brethren, stand fast; and hold the traditions which you have learned, whether by word, or by our epistle.”
2 Thessalonians 3:6: “And we charge you, brethren, in the name of our Lord Jesus Christ, that you withdraw yourselves from every brother walking disorderly, and not according to the tradition which they have received of us.”
Acts 20:35: “I have showed you all things, how that so labouring you ought to support the weak, and to remember the word of the Lord Jesus, how he said: It is a more blessed thing to give, rather than to receive.”
This one is interesting because here St. Paul is referring to words of Jesus that have not appeared anywhere in the four Gospels. Another example of oral traditions being passed down outside of the Scriptures is found in the next verse:
St. Jude 1:9: “When Michael the archangel, disputing with the devil, contended about the body of Moses, he durst not bring against him the judgment of railing speech, but said: The Lord command thee.”
Protestants who believe they can interpret the Bible on their own without an infallible guide, or who think that the Holy Spirit guides every believer to the Truth (despite the contradictions in the beliefs of the thousands of denominations) have not read the following verses, or they simply ignore them out of dishonesty.
2 St. Peter 1:20: “Understanding this first, that no prophecy of scripture is made by private interpretation.”
Acts 8:30-31: “And Philip running thither, heard him reading the prophet Isaias. And he said: Thinkest thou that thou understandest what thou readest? Who said: And how can I, unless some man show me? And he desired Philip that he would come up and sit with him.”
2 St. Peter 3:16: “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.”
So we see that the Scriptures do indeed require someone to teach them who is protected from error. Does the Bible tell us who that should be?
We have seen that St. Peter was made the rock upon which Christ would found His Church in Matthew 16:18. It should be noted that the Apostles were the first bishops of the Catholic Church, even Judas was thus considered when the Apostles number Matthias among them.
Acts 1:20: “For it is written in the book of Psalms: Let their habitation become desolate, and let there be none to dwell therein. And his bishopric let another take.”
1 Timothy 3:1-2: “A faithful saying: if a man desire the office of a bishop, he desireth a good work. It behoveth therefore a bishop to be blameless, the husband of one wife, sober, prudent, of good behaviour, chaste, given to hospitality, a teacher,”
(The clergy were at one time allowed to marry)
Titus 1:7: “For a bishop must be without crime, as the steward of God: not proud, not subject to anger, not given to wine, no striker, not greedy of filthy lucre:”
1 Peter 2:25: “For you were as sheep going astray; but you are now converted to the shepherd and bishop of your souls.”
And St. Peter himself, being the head bishop of the Church, is the only one to whom was given the gift of infallibility by God.
St. Luke 22:31-32: “And the Lord said: Simon, Simon, behold Satan hath desired to have you, that he may sift you as wheat: But I have prayed for thee, that thy faith fail not: and thou, being once converted, confirm thy brethren.”
The word infallible means cannot fail. After this, our Lord put St. Peter in charge of the whole Church.
St. John 21:15-17: “When therefore they had dined, Jesus saith to Simon Peter: Simon son of John, lovest thou me more than these? He saith to him: Yea, Lord, thou knowest that I love thee. He saith to him: Feed my lambs. He saith to him again: Simon, son of John, lovest thou me? He saith to him: Yea, Lord, thou knowest that I love thee. He saith to him: Feed my lambs. He said to him the third time: Simon, son of John, lovest thou me? Peter was grieved, because he had said to him the third time: Lovest thou me? And he said to him: Lord, thou knowest all things: thou knowest that I love thee. He said to him: Feed my sheep.”
It has been clearly demonstrated using only the Bible that justification by faith alone and "once saved, always saved" are lies, that denominationalism is a lie and that there is one Church having Christ as its head. This is the Holy Catholic Church, whose apostolic traditions are to be followed and obeyed equally with Scripture, since they are both complimentary, and the Lord willed St. Peter and his successors to remain on earth as the visible principle of unity and authority, capable of infallibly defining the meaning of both Scripture and tradition. You need to learn the true Catholic Faith, and convert to it, or you cannot attain salvation.