“They were demonstrably not Protestant because Protestantism believes that salvation is through faith alone, and the scripture says the exact opposite.”
The problem with that statement is when Paul is writing to Jewish believers who taught that salvation is through faith plus works, he corrected them with the following,
Rom. 9:30-32, “What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith.But Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. Wherefore? Because they sought it not by faith, but as it were by the works of the law.”
Rom. 10:2-4, “For I bear them record that they have a zeal of God, but not according to knowledge. For they being ignorant of God’s righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God. For Christ is the end of the law for righteousness to every one that believeth.”
Rom. 10:8-13 “The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach; That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. For the scripture saith, Whosoever believeth on him shall not be ashamed. For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him. For whosoever shall call upon the name of the Lord shall be saved.”
Blue Duncan: when Paul is writing to Jewish believers who taught that salvation is through faith plus works, he corrected them with the following [Rom 9-10 selected verses]
More precisely, St. Paul was confronting a belief that a Christian convert should be circumcised. It is circumcision, and more broadly, the belief in the salvific nature of the Hebrew Law that is addressed and demolished in Romans (and Galatians). Since you brought up Romans 9 and 10, let us examine the context. The Jews, he says, "sought justice" "as it were of works" and of course they did not get justice. The Gentiles however, "followed not after justice, have attained to justice, even the justice that is of faith". Note that the distinction is not between works in general and faith, but works of Jewish law and faith. It becomes clear when St. Paul concludes: "there is no distinction of the Jew and the Greek: for the same is Lord over all, rich unto all that call upon him".
So it is faith that erases the distinction between a Jew and a Gentile as either one can believe the same thing. It is the faith that they share that brings salvation to both Jew and Greek. But does it say that it is faith as opposed to good works of charity? Not at all: it goes on to say "But all do not obey the gospel" and later, "if thou abide in goodness, otherwise thou also shalt be cut off" (Rom 11:22). These two chapters teach what the Catholic Church teaches: that faith is necessary for salvation, but the faith must result in obedience of the gospel, -- the works. And indeed, the letter does not end there. Instead, we hear this:
[1] I BESEECH you therefore, brethren, by the mercy of God, that you present your bodies a living sacrifice, holy, pleasing unto God, your reasonable service. [2] And be not conformed to this world; but be reformed in the newness of your mind, that you may prove what is the good, and the acceptable, and the perfect will of God. [3] For I say, by the grace that is given me, to all that are among you, not to be more wise than it behoveth to be wise, but to be wise unto sobriety, and according as God hath divided to every one the measure of faith. [4] For as in one body we have many members, but all the members have not the same office: [5] So we being many, are one body in Christ, and every one members one of another.[6] And having different gifts, according to the grace that is given us, either prophecy, to be used according to the rule of faith; [7] Or ministry, in ministering; or he that teacheth, in doctrine; [8] He that exhorteth, in exhorting; he that giveth, with simplicity; he that ruleth, with carefulness; he that sheweth mercy, with cheerfulness. [9] Let love be without dissimulation. Hating that which is evil, cleaving to that which is good. [10] Loving one another with the charity of brotherhood, with honour preventing one another.
[11] In carefulness not slothful. In spirit fervent. Serving the Lord. [12] Rejoicing in hope. Patient in tribulation. Instant in prayer. [13] Communicating to the necessities of the saints. Pursuing hospitality. [14] Bless them that persecute you: bless, and curse not. [15] Rejoice with them that rejoice; weep with them that weep.
[16] Being of one mind one towards another. Not minding high things, but consenting to the humble. Be not wise in your own conceits. [17] To no man rendering evil for evil. Providing good things, not only in the sight of God, but also in the sight of all men.
(Romans 12)
Further,
now our salvation is nearer than when we believed. [12] The night is passed, and the day is at hand. Let us therefore cast off the works of darkness, and put on the armour of light. [13] Let us walk honestly, as in the day: not in rioting and drunkenness, not in chambering and impurities, not in contention and envy: [14] But put ye on the Lord Jesus Christ, and make not provision for the flesh in its concupiscences.(Romans 13)
We are united in faith, but we are not saved by faith alone. Works of obedience, mercy, charity must accompany our walk of faith. This is what St. Paul wrote, and thsi is waht the Catholic Chruch teaches.