II. Recit et chœur
Beginning at the fifty-seventh bar of the movement (the twenty-sixth bar after the key change that corresponds with the beginning of the soprano solo), the text is "Et hoc vobis signum" ("And this to you a sign" - the first part of Luke 2:12). The musical setting of "vobis signum" forms a cross:
I first learned of this cross-inscribing feature in John Eliot Gardiner's Bach: Music in the Kingdom of Heaven. Gardiner illustrates Bach's use of this in Christ lag in Todes Banden, BWV 4:
In the Christmas account, the "sign" of the Savior for the shepherds is "find[ing] a baby wrapped in swaddling cloths and lying in a manger," but this is also a foreshadowing of the crucifixion, which is also a sign of "a Savior, who is Christ the Lord" (Luke 2:11).
I've found this cross inscription in quite a few pieces now, but I don't know if any of them were placed intentionally (although I can usually devise a plausible meaning for their placement). However, since I first learned about this in a Bach cantata and the first movement of Saint-Saëns' Christmas Oratorio has the note "dans le style de Séb. Bach" ("in the style of Seb[astian] Bach"), I feel that it's more likely that Saint-Saëns - since he was obviously familiar with Bach's work - created the cross figure intentionally.
The chœur section of the second movement has the text "Gloria in altissimis Deo, et in terra pax..." ("Glory to God in the highest, and on earth peace..." - Luke 2:14). "Gloria in altissimis Deo" is first sung by the sopranos and the altos, the higher voices, and "Et in terra pax" is first sung by the tenors and the basses, the lower voices:
(click the image to enlarge it)
Distributing those parts of the text among those particular voices results in an audible representation of the highness of Heaven and the lowness of Earth. (Incidentally, Handel does this same thing [with the same text even] in the Messiah.)
Furthermore, the "altissimis" ("highest") is set across a series of rising notes, as if to further indicate the height:
V. Duo (soprano & bass)
At the 60th bar, the cross inscription is seen again (it's also present a few times in earlier measures, but here it's clearer to see):
Since this is a duet and the text has a predicate nominative, it's easy to set it in such a way that the subject and the predicate nominative are sung simultaneously, which Saint-Saëns does. The grammatical structure is doubled by the musical structure. In the first bar in the example above, as the soprano sings "es tu" ("you are"), the bass sings "Deus" ("God"). Then it's reversed and the soprano sings "Deus" as the bass sings "es tu." Between them, it's "You are God."
IX. Quintette et chœur (soprano, mezzo-chorus, alto, tenor, bass)
A few months ago, I wrote a post about the melody to which some of the "alleluias" here are set and how it resembles a melody that Palestrina wrote.
X. Chœur
I'm not sure if there's a Biblical source for this text either, but the tenth movement begins with "Tollite hostias et adorate Dominum in atrio sancto eius." I put some effort into this translation and came up with "Raise sacrifices and worship the Lord in His holy hall."
Of course, after I did my own translation, I found one on the internet: "Bring offerings, and adore the Lord in his holy place." The "His holy" is what I'm drawing attention to here, because it's set to yet an-other cross inscription, this time in both the soprano and bass parts:
It's not as obvious why the cross is inscribed here as it is in its other occurrences. Since half of it accompanies the first part of "holy," it seems like it has something to do with sanctification. I'm not sure, but I thought I would point it out.
There are actually multiple crosses here. There's one in the soprano part, one in the bass part, each of those is doubled in the corresponding strings, there's one in the the higher register of the organ part (the second line from the bottom), and - though it might be far-fetched - there's also one in the movement number (X).
