Any mass Rome promulgates is legal to attend by virtue of that very fact. And the priests were ordered to say it, see the quote at the end of this post.
Under normal circuмstances, with no wolves in the Vatican, you'd be correct in that assumption. But the law says what the law says and since Trads are aware of the infiltration, we can't just judge all promulgations as being equal. We have to look at what the law says. The conspirators often use our assumptions and naivety against us. The fact is that the law promulgates/legalizes the creation of a missal, but does not legalize the use of it, nor does the law require use of it.
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Correct me if I'm wrong, but I believe Benedict XVI only said John XXIII's missal was never abrogated, and more specifically never abrogated as an extraordinary form. If he said more specifically that Quo Primum wasn't abrogated, please link me the quote, I honestly haven't seen it.
A law cannot be separated from the purpose of the law. Quo Primum's purpose was to create a uniform/universal latin missal. The 1962 missal is the missal of QP. If the 1962 missal isn't abrogated, then QP isn't either. They go hand in hand, because the 1962 missal is just a REVISION of the ORIGINAL St Pius V missal of QP. Benedict did not directly say that QP wasn't abrogated because he, as a conspirator, doesn't want to draw attention to QP. But he also doesn't have to, because it's understood in a legal sense.
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For example: If you signed a lease to an apartment, this gives you the right to piece of real estate. If the lease ends, you can't use the apartment anymore. If you are paying rent, then the lease still exists. You can't use an apartment without a lease and (normally) you don't pay rent unless you can use the apartment. ...You can't have the 1962 missal without QP. If QP is abrogated, then so is the 1962 missal.
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The "ordinary/extraordinary" form in the motu, was Benedict's spin to explain how the 1962 missal should co-exist with the new mass. There is no legal basis for these terms, because they are neither mentioned in QP or in Paul VI's law. They are more novelties.
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Anyway, the reason I think that distinction is important is because saying "it was never abrogated, as an extraordinary form" just means that the rite, in its new and updated state, still exists and is within use in the Church. But that does not necessarily confirm that all the details of Quo Primum remained in place, or that the rite is still the ordinary form.
"it was never abrogated, as an extraordinary form" - This phrase was never used, nor hinted at.
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(QP's) commands were directed at the clergy, I don't think one pope can limit the authority of future popes. For example, QP was altered many times by popes - even if they were only minor changes it still proves that QP's strict order to not alter the rite does not apply to them.
Paul VI had all the power in the world to abrogate QP, but he didn't. He could've revised the 1962 missal and morphed that into the new mass, so that the 1962 missal would cease to exist. But he didn't. He left the 1962 missal and QP as is, and created a new liturgy, which is separate from QP. He did not revise, edit or add to QP, thus, this law is still 100% in force, in all its details.
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Those popes who revised the missal after the creation of QP only revised non-essential elements of the law (i.e. minor changes to the missal). They did not revise or overturn QP's commands, prohibitions and allowances. And every time there was a new missal, the new law mentions that they are non-essentially revising QP's missal. QP is the parent law, and all these revisions are related to it.
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To continue the apartment lease analogy, if you signed a lease which ends on Dec 31 but you agree with the landlord to live there another 6 months, he could ask you to sign an addendum (i.e. a revision) which says you promise to lease for another 6 months. You wouldn't need to sign a brand-new lease; you would sign a 1 page docuмent which refers back to the original lease. ...In the same way, when all these revised missals were created, they all referenced QP. There is a legal chain-link from 1962 all the way back to the 1570s.
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Paul VI's missal was a brand new law, a brand new missal. It has no connection with QP, and Benedict makes this clear. Benedict only argues that the new mass is a different "usage of the same rite". I don't even know if canon law has a definition for "usage" when it comes to rites. This sounds made up (the Modernists love to make up new words). QP does not allow the use/attendance of any other missals or "usages", under pain of sin.
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Paul VI) ordered that it be used and he ordered it go into effect on the first Sunday of Advent that year.
In conclusion, we wish to give the force of law to all that we have set forth concerning the new Roman Missal. In promulgating the official edition of the Roman Missal,
...We order that the prescriptions of this Constitution go into effect November 30th of this year, the first Sunday of Advent.
No, Paul VI did not order anyone to use his missal. He said "we give the force of law to all that we have set forth concerning the new missal"...what did he set forth? He created a new missal, he explained the changes in it and that's it. He did not say who was to use it, who HAD to use it, nor if there were penalties for NOT using it.
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"We order that the prescriptions (commands) of this Constitution go into effect Nov 30..." What are the commands of this law? Only that a new missal be created. Nothing more.
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Modernists knew they could not legally force people to attend a new mass while QP was in force. They knew that they could not revise QP with the new mass, because such changes were essential changes and God would not allow the Church to pass a law which forces sin. They knew that they could not abrogate QP and replace it with a theologically anti-Catholic liturgy. So what did they do? They created a new missal and let everyone assume that they had to attend, even if the law said otherwise. They knew the pope could not force people to attend, but the bishops/priests could, because rogue/evil bishops are not protected by infallibility. The bishops/priests are the ones who forced the new mass on the people, not rome/pope. The V2 popes have certainly supported and said the new mass, but they did so in a non-official, non-legal manner. They sinned in their personal capacity. They did not force the new mass on anyone by law or by penalty of sin.
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There's no specific penalty specified that I can see, but ignoring the missal would still be disobedience for priests, so the penalty is sort of built-in in that respect.
QP specifically says that no clergy (and by extension the laity) can be forced to say any other missal, other than QP's (1962). Even if the order comes from bishops, cardinals or any other official. QP clearly gives all priests the permission (and in another section, it is a command) to say the latin mass "in perpetuity". Benedict's XVI's motu confirms this permission and reiterates that all priests have the right to the latin mass, because it was never abrogated and consequently "always permitted".
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Ergo, this confirms that all the evil Bishops/priests/superiors of the 70s, 80s, 90s were legally wrong and that those who "disobeyed" to stick with Tradition did correctly. They obeyed the pope (St Pius V) and the law when their superiors commanded sin.