In December, Fr. Cassian Folsom, OSB delivered a talk at the Brompton Oratory in London on Summorum Pontificum and liturgical law. Fr. Folsom by way of background is the abbot of the monastery of Norcia, Italy, a house specifically asked by the Holy Father Pope Benedict XVI to celebrate well both the usus antiquior and the Novus Ordo.
The text has just been released and can be found HERE. His talk, as he noted, was not so much on the specifics of the law but on the general legal situation which allows two Roman Missals to co-exist. If the principle lex orandi, lex credendi is true, then how do we have a united lex credendi if the lex orandi in each form is very different even to the point of its theologies?
It's fine for the purposes of ecclesiastical law to begin in 1570, when Pope St. Pius V issued the bull Quo Primum issuing the revised Missale Romanum as ordered by the Council of Trent and suppressing those books in use which were not of two hundred years' antiquity. I realize too that Fr. Folsom's audience members are probably well-aware of these things, so this isn't a criticism of his talk: rather it's an attempt to contextualize everything a bit more. The Roman Rite is very old. It was fixed by the seventh century, more or less, and the prayers go back to the third. The intercessions for Good Friday are a good example of such prayers. In fact, I was just reading a blog entry wherein it was asserted that a prayer in the older form was likely written by Pope St. Leo the Great. Alas, I can't recall where it was or which prayer... In the newer form, the prayers are rearranged from the ancient sacramentaries, although the developments of the centuries in-between tend to be garbled, an issue to be addressed later. Alas, I can't recall where it was or which prayer...
Father John Hunwicke has a little test, called the Stowe Test, after the Irish Stowe Missal from the late eighth , century. One sees the development of liturgy, which should as much as possible be locally generated before spreading to the universal Church formally, on display. The Filioque in the Credo and the Offertory prayers, compiled from Gallican, Mozarabic, and Roman sources, are very good examples of this rule. They were not passed down by the Pope for the sake of inserting something new. A good exception to this rule is the Missa Signum Magnum for the Assumption, promulgated at the time the dogma was declared in 1950. The Missal of 1474, as Fr. Hunwicke pointed out, is still serviceable, so long as one has binding adhesive for new Masses and prefaces and a pencil. (St. Joseph in the Communicantes comes to mind.)
This was the first printed edition of the Missal, a very nice development, since the Franciscans had spread the liturgies of the Papal Court throughout all Europe. The major additions of the 1570 book, ones noticeably absent from the Pauline rituals, are the Praeparatio (Psalm 42 Judica Me, etc.) and the Last Gospel from John. These were made a part of the liturgy in recognition of their worthy and widespread use. Medieval parishes did not have sacristies, so the priest prayed, vested, said Mass, and removed his vestments all in one space. We should not treat this accident of history lightly or too heavily, however, because the Prayers at the Foot of the Altar and the Last Gospel work so well in the liturgy. These additions were organic changes, nor were they added to the liturgy before they were part of the liturgy. As to deletions, most of the admittedly very beautiful sequences were pruned from the Missal since they were extremely late additions to the Mass, all after the ninth century until the Protestant Revolt. (More were retained in the Dominican Rite, the proper religious calendars, and I suppose the medieval uses-Sarum, York, etc.-as well.) Compare this to changes made willy-nilly by priests in the last few decades. At least the fact they were composed for the occasion, even on the spot, means they won't be added to the Missal.
It's quite interesting that one can see this care for the liturgy in the old books. One shouldn't treat things used for sacred purposes carelessly, of course, but when printing is expensive (or non-existent) one needs to be extra-careful so that when changes are necessary the whole book isn't thrown out without any hope for replacement. I believe this is the best representation of the way the Church and all her faithful should treat the sacred liturgy.