When taught to translate English into Greek or Latin (something I was never very good at) I was told to use the idioms of a classical author appropriate to the genre of the text. For example, if the text prescribed for translation came from a speech of Churchill, one would go to Cicero or Demosthenes. In many cases any prose writer would do, but if a piece of grammar only occurred in the poets then it was not to be used. At Merton we all had a terrible time with trying to translate something from Proust.
What would be the equivalent for somebody ordered to provide a translation into Latin of an extract from the works of Blessed John Henry Newman (for example) for his office? In other words what counts as Ecclesiastical Latin? Somewhere in volumes iii or iv of Liturgia Horarum is a sermon by St Leo the Great which uses a grammatical construction not found in classical authors. I'll track it down later but, for the purpose of this post, it is enough to remark that Cicero and co. need not be our only models for writing Latin now.
Leaving aside the question of models – examples of good style – if we want to know if something is Ecclesiastical Latin, what are the sources of the Language? An obvious such source would be the Vulgate and its predecessors – bearing in mind that some constructions are literal translations of the Hebrew. If you wanted to describe something made of individual parts as strongly compacted together, you would not say "cuius participatio eius in id ipsum" (Ps 121(122):3). Later on Jerome rendered that passage "cuius participatio eius simul". Pius xii gave us the elegant "in se compacta tota", but these days the Church has compromised with "sibi compacta in idipsum".
Presumably Jerome's other works are sources, as are those of Tertullian, Cyprian, Augustine and Ambrose. St Irenaeus of Lyons wrote in Greek, but his only works to survive intact do so in Latin translations made during his lifetime. Presumably they could be a model. You would probably include Aquinas, Bernard of Clairvaux and Bernardine of Siena as well as all Papal documents at least up to the middle ages.
Veni sancte Spiritus, the Sequence for Pentecost, was written in the 12th century, probably by Innocent III (ob.1216) or by his friend from the Sorbonne, Stephen Langton, Archbishop of Canterbury. I would be inclined to include the acts of the council of Trent (not meaning to exclude anything else up to that point) as well as the Breviary (1569) and the Missal (1570).
Urban VIII (1623-1644), assisted by some learned Jesuits, replaced many of the hymns of the Breviary with new versions in a more Classical language and metre. These survived until 1969. The Catholic Encyclopedia remarks that
surprise may be expressed at the temerity that dared to meddle with the Latinity of a Prudentius, a Sedulius, a Sidonius Apollinaris, a Venantius Fortunatus, an Ambrose, a Paulinus of Aquileia, which, though perhaps lacking the purity of the Golden Age, has, nevertheless, its own peculiar charm. Even the more barbarous Latinity of a Rhabanus Maurus is not without its archaic interest and value.
But are these Classicised hymns Ecclesiastical Latin? And can they be used as models for modern compositions? Maybe but perhaps, by definition, no. How about the 1917 code of Canon Law? Perhaps. How about the 1983 code, composed from scratch in my lifetime (perhaps a little earlier)? The mere fact that an idiom is in St Leo the Great means it could be used in translating (say) John Henry Newman's sermon on the Roman See. What if it is in Vatican II and nowhere else?
I see very little evidence that any thought whatsoever has been given to this sort of question. Instead people are left to do their work with whatever Latin they happen to have picked up along the way. We end up with things like this, from the Office of Readings of Padre Pio (23rd September), translated from one of his letters (presumably in Italian):
Gratias, ergo, agite infinitæ pietati æterni Patris, qui sic animam vestram ad salutem deputatam gerit. Cur non gloriari benevolis his optimi ex omnibus patribus adiunctis?
I cannot construe the second sentence. I cannot see how dative plural benevolis his connects with omnibus patribus adiunctis nor what genitive singular optimi is doing unless it is nominative plural and subject of some understood verb. Perhaps somebody believes benevolo is a verb and that benevolis is its second singular.
Via New Advent, Msgr Daniel Gallagher – apparently the successor to Fr Reggie Foster as one of the Pope's Latinists – discusses the return of Latin in the Church.