In my commentary on the other Psalm of Friday Lauds in the traditional Benedictine Office, Psalm 75 (76), I suggested that its selection reflected its clear allusion to the events of Good Friday, particularly the reference to the earthquake that occurred at the hour of Our Lord's death on the Cross.
I have to say though that for a long time I was fairly puzzled about the reasons for the inclusion of Psalm 91(92) on Friday. It certainly contains allusions to the Crucifixion, but overall it is a rather joyous hymn; indeed its title suggests that in the Jewish tradition it was said on the sabbath (ie Saturday), and indeed the Old Roman Office retained that position for it.
Christ's sacrifice replaces those of the Temple
Eminent Orthodox scholar Patrick Reardon, in his book Christ in the Psalms, however, has provided an elegant and plausible solution to this puzzle, for he notes that as well as the Sabbath, Jewish commentaries state that it was sung daily as an accompaniment to the daily morning sacrifice of a lamb. Reardon, accordingly, sees the shift of the psalm to Friday Lauds as a testimony to the idea that Friday is "our true the true Pascha and Atonement Day, on which the Lamb of God took away the sins of the world."
He sees Psalm 91 as a reminder that the Old Covenant, which merely foreshadowed what was to come, has ended, and the New has replaced it:
"Prayed on Friday mornings, as the ancient Western monastic rule prescribed, this psalm reminds the Church why it is no longer necessary to make the daily offering of lambs in the temple, for those sacrifices had only "a shadow of the good things to come, and not the very image of the things" (Heb. 10:1). With respect to those quotidian lambs offered of old, we are told that "every priest stands ministering daily and offering repeatedly the same sacrifices, which can never take away sins" (10:11). But, with respect to the Lamb in the midst of the Throne, we are told that "this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God . . . For by one offering He has perfected forever those who are being sanctified" (10:12-14). This is the true Lamb to whom we chant: "You are worthy to take the scroll, / And to open its seals; / For You were slain, / And have redeemed us to God by Your blood" (Rev. 5:9)." (p181)
St Benedict on the Old Covenant
Is it plausible that St Benedict was aware of the Jewish tradition? Sociologist Rodney Stark has drawn attention, in a number of his books on the early Church, on the close relationship and competition between Jewish and Christian communities in the early Church. Certainly there is a large volume of Patristic literature which St Benedict would have had access to, directed against the Jews that is plausibly explained by the problem of relapsing/Judaizing Christians. And there was also a lot of other material on Jewish culture available at the time: Cassiodorus attests, for example, that Josephus' Antiquities for example was available in Latin at this time. The idea that St Benedict would deliberately shift this psalm out of Saturday as something of a statement on the Old Covenant is also supported, I think, by two other instances in the design of his Office where I think St Benedict may be having a subtle poke at the Jews. One instance concerns Psalm 118, which the traditional Roman Office gets through in a day, but St Benedict spreads over Sunday and Monday. St Benedict ends Sunday, the eighth day's, segments of the psalm with the psalmist claiming to have outshone his teachers and those of old in his understanding.
The second case also has to do with the Sunday Office: on Sundays he sets Psalm 117 at Lauds and ends Vespers with Psalm 112. These are the last and first respectively of the Hallel psalms, songs of praise used on Jewish festivals. A kind of coded allusion to the promise of their eventual conversion in that the first shall be last and the last first?
The scandal of the Cross
In any case, if the overall theme of the day is Christ’s sacrifice on the Cross, in this psalm, I think we are called on to contemplate the deep mystery of God’s plan (vs 5). The fool, the psalmist states in verse 6, fails to understand: to him, St Paul points out, the Cross is a scandal. Yet the Cross enables all of us to be reconciled to God through Christ. Indeed, the Fathers interpreted verse 10, talking about the exaltation of the horn of the unicorn, as a direct reference to Christ’s crucifixion and resurrection. Horned animals were sacrificed to God, as Our Lord became the Lamb of God on the Cross.
St Benedict's overall take on Good Friday though, is a relatively upbeat one, I think, focused on the promise of the Resurrection rather than dwelling unduly on the Cross.
And if his move of this psalm from the Jewish Sabbath to Friday is something of a statement, it is one with a note of hope in it as well, for St Benedict was surely aware that St Paul (Rom 11:33) quotes verse 6 of the psalm immediately after his prophesy of the ultimate reconciliation of the Jewish people to Christ.
Psalm 91 (92): Bonum est confiteri Dominum
Psalmus cantici, in die sabbati.
A psalm of a canticle on the sabbath day.
1 Bonum est confitéri dómino: * et psállere nómini tuo, altíssime.
It is good to give praise to the Lord: and to sing to your name, O most High.
2 Ad annuntiándum mane misericórdiam tuam: * et veritátem tuam per noctem
3 To show forth your mercy in the morning, and your truth in the night:
3 In decachórdo, psaltério: * cum cántico, in cíthara.
4 Upon an instrument of ten strings, upon the psaltery: with a canticle upon the harp.
4. Quia delectásti me, Dómine, in factúra tua: * et in opéribus mánuum tuárum exsultábo.
5 For you have given me, O Lord, a delight in your doings: and in the works of your hands I shall rejoice.
5 Quam magnificáta sunt ópera tua, Dómine! * nimis profúndæ factæ sunt cogitatiónes tuæ
6 O Lord, how great are your works! your thoughts are exceeding deep.
6 Vir insípiens non cognóscet: * et stultus non intélliget hæc.
7 The senseless man shall not know: nor will the fool understand these things.
7 Cum exórti fúerint peccatóres sicut fœnum: * et apparúerint omnes, qui operántur iniquitátem.
8 When the wicked shall spring up as grass: and all the workers of iniquity shall appear:
8 Ut intéreant in sæculum sæculi: * tu autem Altíssimus in ætérnum, Dómine.
That they may perish for ever and ever: 9 But you, O Lord, are most high for evermore.
9 Quóniam ecce inimíci tui, Dómine, quóniam ecce inimíci tui períbunt: * et dispergéntur omnes, qui operántur iniquitátem.
10 For behold your enemies, O lord, for behold your enemies shall perish: and all the workers of iniquity shall be scattered.
10. Et exaltábitur sicut unicórnis cornu meum: * et senéctus mea in misericórdia úberi.
11 But my horn shall be exalted like that of the unicorn: and my old age in plentiful mercy.
11 Et despéxit óculus meus inimícos meos: * et in insurgéntibus in me malignántibus áudiet auris mea.
12 My eye also has looked down upon my enemies: and my ear shall hear of the downfall of the malignant that rise up against me.
12 Justus, ut
13 The just shall flourish like the palm tree: he shall grow up like the cedar of Libanus.
13 Plantáti in domo Dómini, * in átriis domus Dei nostri florébunt.
14 They that are planted in the house of the Lord shall flourish in the courts of the house of our God.
14 Adhuc multiplicabúntur in senécta úberi: * et bene patiéntes erunt, ut annúntient:
15 They shall still increase in a fruitful old age: and shall be well treated, 16 that they may show,
15 Quóniam rectus Dóminus, Deus noster: * et non est iníquitas in eo.
That the Lord our God is righteous, and there is no iniquity in him.