The argument goes thusly: Since God Himself did not find it necessary to institute an infallible teaching authority under the Old Covenant, the Roman Catholic argument is thereby highly suspect unless it has exegetical support. Put another way:
If Christians require the services of a living Magisterium, wouldn’t the Old Covenant community be under the same necessity? Yet it’s clear from the Gospels that none of the rival parties spoke for God in any definitive sense. The priesthood was the only faction with any institutional standing under the Mosaic Covenant, and its members were frequently and fundamentally mistaken in their construal of its ethical obligations, such as the matter of putting to death their prophesied Messiah. So much for a divine teaching office to ensure unity and fidelity.
The typical Roman Catholic reply is rather too often something like this:
“In OT times, the Jewish assembly was not yet given the gift of infallibility. Things change after Jesus comes and the Holy Spirit indwells believers. Is this not elementary?”
Apparently, the argument has gone over Dave's head, so it appears we'll have to break it down for him.
1. His response assumes a great deal that it needs to prove. This should go without saying.
2. The Protestant argument is twofold:
a. It goes to God's historical modus operandi. Why is what was acceptable for God’s old covenant community unacceptable for God’s new covenant community? If you want to know God’s intentions for the future (e.g. the church age), a good place to start is with his historical modus operandi.
b. It is also exegetical:
3. Apropos 2b, a few notes:
a. Old Covenant polity and Roman Catholic polity parallel on several levels
i. The priestly class (the Magisterium) and a high priest (Pope).
ii. Mediate authority between God and man via this class.
iii. Outward mediation of grace via the Roman sacraments parallels the sacrificial system (the Eucharist), circumcision (baptism), Levitical priesthood (holy orders), feast days, etc.
iv. The Temple, some Romanists might say, would parallel the Vatican itself.
So, the Catholic who offers the argument for infallibility wants on the one hand to maintain some continuity with the Old Covenant's outward structures (including such things a Mary being a "Queen Mother"), but the appeal is, in reality, quite selective. The Protestant argument from the Old Covenant is, therefore, a way of answering the Romanist on his own level.
Of course, the Bible itself, namely Hebrews, is quite clear that the Old Covenant is simply signs and shadows of what was to come (the New Covenant). The New Covenant is also said to be disanalogous on several levels. For example, there is no more high priesthood except for that of Christ (Hebrews). The church as a whole is a holy nation, a royal priesthood, etc. (1 Peter), eg. a kingdom of priests (Hebrews 1:16, which - by the way is the fulfillment of Exodus 19, so this is also a point of analogy as well as disanalogy, for Israel ultimately failed), feast days and the Jewish liturgical calendar are abolished (Col. 2:16, 17), and, other than the Lord's Day no other liturgical calendar is given in Scripture.
b. There is no infallible teaching authority under the Old Covenant, and, if the Romanist is going to make this argument, then the Romanist needs an exegetical argument that there is some sign / shadow in the Old Covenant that prefigures the need for an infallible authority in the New Covenant. It's not enough to say, "the gift of infallibility was not yet given," because:
i. The Bible nowhere names a spiritual gift called "infallibility," in the NT or in OT prophecy and
ii. The spiritual gifts in the New Covenant find analogues in the Old Covenant.
The Romanist can say, something like "the Levitical caste decided between difficult cases under the Law" - but this is not the same as saying "they were infallible teachers." They can say "Prophets spoke infallibly from God," but the Protestant simply replies that the Prophets, as to office, are analogous to the Apostles, and we have their extant teaching already. Thus, the Romanist makes the Pope the successor of Peter, and on and on the argument goes - yet what is always missing in the exegetical link to the need for and the promise of an infallible teaching office in the New Covenant.
Further, one of the discontinuities between the Old and New Covenants is that the mediate "gifts" like prophecy and teaching are given to individuals within the body of Christ as a whole, not a specific mediating office like "priest" or "Pope."
Thus, another of the points of disanalogy between covenant epochs is that these gifts are dispersed throughout the people of God widely, not concentrated within one or two classes of persons (priests, prophets). Teaching is immediate in the sense of not being mediated so much by an office as through individuals who are themselves from all "classes" and genders within the church. In fact, the intelligent Protestant would point out that the very office of "prophet" in the Old Covenant prefigures this distribution of gifts widely, for the Prophets were not all concentrated in the school of Samuel or Elisha, you know. They were not all priests or kings; rather the bulk of them were common people, what we'd call "laity" by today's standards. The exact level of dispersion will, of course show up with great variation within local churches - since not every local church in the same. Romanism, by way of contrast, comes off as trying to make the disperson of gifts artificially uniform. The contrast with Scripture could not be more stark.
Further "infallibility" is admittedly a "gift" in Romanism that is restricted to a specific class, for the "infallibility" of the "Church" (eg. the Roman Church - a visible, ecclesiastical body) is dependent on the Magisterium and the Pope - a specific class and a specific office. The Romanist who says that "the Church" is infallible can only do so by reference to these offices. This flies in the face of what was said by (ironically) Peter himself in his Pentecostal sermon:
7" 'In the last days, God says,
I will pour out my Spirit on all people.
Your sons and daughters will prophesy,
your young men will see visions,
your old men will dream dreams.
18Even on my servants, both men and women,
I will pour out my Spirit in those days,
and they will prophesy.
And Paul's own discourse on gifts (1 Cor. 12, Eph. 4) does not lead us to the conclusion that the relevant gifts (teaching/prophecy) are restricted to a particular ecclesiastical class - and nowhere does he ever talk about the need for "infallibility."
In fact, he says that knowledge of sound doctrine is recognized and maintained not by a guarantee of infallibility that is attached to an office that administers particular gifts - rather, Paul speaks about division in the church in positive and negative light.
1 Cor. 11:18-19, "For, in the first place, when you come together as a church, I hear that divisions exist among you; and in part, I believe it. 19 For there must also be factions among you, in order that those who are approved may have become evident among you."
1 Cor. 1:10, "Now I exhort you, brethren, by the name of our Lord Jesus Christ, that you all agree, and there be no divisions among you, but you be made complete in the same mind and in the same judgment."
1 Cor. 11:19 uses the Greek word "haireses" for "factions". "Heresy" is a false teaching, something that deviates from orthodoxy. If we see that the Scriptures declare something clearly (orthodoxy), and if someone teaches contrary to that clear teaching, then he or she is teaching heresy. The Scriptures teach that there is a place for division and that is when opposing teachings that are contrary to sound doctrine. But division can only occur when the truth is known and those who abide with the truth should correct those who do not. If there was one place for Paul to talk about the need for an infallible arbiter of truth, here is where it should be. It is nowhere to be found.
The point, of course, is that the place where one would expect to find it: the Old Covenant system does not have it, and the New Covenant widely distributes the gifts without distinction, which flies in the face of the Romanist's own logic, for he can't argue that, because of this wide distribution there is a new need generated - an infallible teaching office- only to restrict it, for the appeal to infallibility requires the restriction of - not the wide distribution of - the relevant gift. Thus, he's left to argue, if he chooses that route, for the wide distribution of a gift like teaching among the people generally but the restriction of the gift of infallibility to an office, but this is self-defeating, for the gift of infallibility would have to be attached to the gift of teaching in order to be of any use or effect.
Now, the Roman Catholic can argue that the clergy has the effect of concentrating these gifts into particular offices - but the Protestant can agree to this much - for, with a few exceptions like Quakers, we ordain or appoint ministers of the gospel (elders) and deacons and try to employ our members who manifest the teaching gift to that ministry either formally or informally in the churches. In addition, most of us recognize there are teachers that exist outside of these specific offices (elder/deacon), which seem to serve the purpose of calling those in those offices to account from time to time. Many a Protestant minister (and Catholic priest) can tell of the "wise woman" or "incredibly intelligent young man" who sits in their church very humbly, doesn't call attention to himself or herself, and yet is somebody to whom that minister can go to for wise counsel, to help them study, and who will even challenge their teaching from time to time - yet in terms of ecclesiastical office, they submit themselves to the authority of the elders and deacons of the local church as prescribed in Scripture.
Further, if the Old Covenant is greater than the New Covenant, and the New Covenant is written on the heart of the people (Jeremiah 31), why would there need to be an infallible teaching authority? In fact, the Protestant argument is that one of the things that makes the New Covenant "new" is not the writing of the Law on the heart via regeneration or justification by faith alone via grace alone (and certainly not the need for or gift of an infallible teaching office) but things like:
1. The extension of salvation from a single visible nation, Israel, to "the whole world." The curse of the Diaspora upon the covenant community is reversed in the New Covenant - it becomes a blessing (1 Peter). The curse of Babel is reversed (Acts 2).
2. The revelation of the mystery of (1) and the making plain of the fact that this is how this has been how men have been saved all along (Romans).
3. The distribution of teaching gifts outside of the classes of "prophet" and "priest."
He can say that an infallible teaching office is "better" than not having one, but how does this follow? If the covenant is "better" as a whole, why would it require an infallible teaching authority? Where is the supporting argument from for the need for such a thing? Wouldn't it make more sense to have an infallible teaching authority for a covenant written on tablets of stone and than one one written on hearts of flesh, and if one of the hidden mysteries of the whole covenant of grace is, in fact, that it has been written on tables of flesh all along, in the sense of individual salvation being via the same means - faith alone and grace alone - and the Old Covenant did not require an infallible external teaching office then, why would it now need one? That's what we're really talking about anyway isn't it - an external teaching office? This isn't about the phenomenology of the inspiration of Scripture or the illumination of the mind, rather it's about an external teaching office, and that too carries over between covenants - both have an external teaching office both include a teaching/prophecy component, but in the Old Covenant the office and gift is restricted; in the New Covenant it is widened. The responsibility of the teaching office itself continues - as elders - but the gift itself is not limited to the eldership.
The Catholic can point out that the Old Covenant teaching authority "failed," but how is this an argument for the necessity of an infallible teaching office under the New Covenant?
1. Wasn't this "failure" precisely what precipiated the institution of the New Covenant? Matthew, after all, among other things, presents Jesus bringing a lawsuit from God upon the nation - faulting the priestly class. Since the New Covenant was prophesied in Jeremiah, isn't this precisely the intention of the Old Covenant's "failure?" If the OT rule of faith "failed," that is not because it was flawed. To the contrary, if it failed, then it did so because it was designed to do so. In fact, the OT rule of faith was a success. It succeeded in achieving the purpose that God meant for it.
2. Signs and shadows are just that. The sacrifices did not take away sin either - and they were never intended to do so. Old Covenant believers had their sins taken away via the cross, not bulls and goats. Likewise, in the Old Covenant, promises are made to Abraham for example, and yet they are not all fulfilled until Christ.
3. The Bible tells us about a wholly sufficient, "infallible" sacrifice offered once for all time (Hebrews) - Christ's - but makes none for an infallible teaching office. Note that on the one hand, the Catholic wants to maintain an infallible teaching office but wants to deny the once for all, complete sufficiency of Christ's sacrifice by the perpetual sacrifice of Christ "on a thousand altars" in that abomination called the Mass, and this must be administered by a priest in a valid holy order, etc. Again, the contrast could not be more stark.