The third psalm of Monday Vespers in the Benedictine Office is Psalm 115.
Psalm 114 opened with words 'I loved'; Psalm 115 (with Psalm 116 to which it is joined under the same Gloria). That opening perhaps points to the three psalms forming something of a triptych within Monday Vespers by recapitulating the key themes of Psalm 113, starting from its opening word, Credidi, or 'I believed'.
One can also, I think, see echoes, in the structure of Monday Vespers, of the progression of ideas presented to us at Prime (and echoed again at Terce), for Psalm 1's presentation of the perfect man has parallels in Psalm 113&114, while Psalms 115&116 contains echoes of Psalm 2.
The reference in verse 4 to the chalice of salvation gives this psalm a strong Eucharistic flavour, and accordingly it has long been used as part of a priest’s preparatory prayers for Mass and for the priest’s communion in the Traditional form of the Mass. For similar reasons, one assumes, it is also used at Tenebrae for Maundy Thursday.
Yet placed in the context of the Benedictine Office, the primary focus becomes instead, I would argue, a further meditation on our response to belief, above all in monastic profession and the performance of the Opus Dei.
I have previously suggested that there are two key sets of vows or promises alluded to in today's Office, namely our baptism (Psalm 113), and in the Benedictine context, monastic profession (or oblation), with the Suscipe verse of Psalm 118 said at Terce.
Psalm 115 brings us back to this theme by pointing to the sacrificial offerings we render to God in response to the good things he has given us (Verse 3). In particular, there is the sacrifice of the Mass, or the 'chalice of salvation' of verse 4. But secondly, there is the 'sacrifice of praise' that is above all the Divine Office (verse 7) offered publicly by the monk (Verse 8).
Yet what is actually asked of us is not just particular separate sacrifices, but rather a complete offering of ourselves, as St Paul in 2 Cor 4:13:
For while we live we are always being given up to death for Jesus' sake, so that the life of Jesus may be manifested in our mortal flesh. So death is at work in us, but life in you. Since we have the same spirit of faith as he had who wrote, "I believed, and so I spoke," we too believe, and so we speak, knowing that he who raised the Lord Jesus will raise us also with Jesus and bring us with you into his presence.
Indeed, Pope Benedict XVI suggests that the verse points to the total holocaust of self:
"How then, shall I make a return to the Lord'? Not sacrifices nor holocausts... but my entire life itself. For this he says: "I will lift up the cup of salvation', giving the name "cup' to the suffering of spiritual combat, of resisting sin to the point of death; besides, that is what our Saviour taught us in the Gospel: "Father, if it is possible, let this cup pass me by'; and again to the Apostles: "Can you drink the cup I shall drink?', clearly symbolizing the death that he welcomed for the salvation of the world" (PG XXX, 109), thus transforming the sinful world into a redeemed world, into a world of thanksgiving for the life the Lord gives us.
For the monk or nun, that holocaust, that white martyrdom, consists of the the embrace of the evangelical counsels of poverty, chastity and obedience in their fullest form, providing the whole Church with a living icon of Christ. As Pope John Paul II said in Vita Consecrata 33:
"A particular duty of the consecrated life is to remind the baptized of the fundamental values of the Gospel, by bearing "splendid and striking testimony that the world cannot be transfigured and offered to God without the spirit of the Beatitudes".The consecrated life thus continually fosters in the People of God an awareness of the need to respond with holiness of life to the love of God poured into their hearts by the Holy Spirit (cf. Rom 5:5), by reflecting in their conduct the sacramental consecration which is brought about by God's power in Baptism, Confirmation or Holy Orders. In fact it is necessary to pass from the holiness communicated in the sacraments to the holiness of daily life. The consecrated life, by its very existence in the Church, seeks to serve the consecration of the lives of all the faithful, clergy and laity alike."
1 Crédidi, propter quod locútus sum: * ego autem humiliátus sum nimis.
2 Ego dixi in excéssu meo: * Omnis homo mendax.
3 Quid retríbuam Dómino, * pro ómnibus, quæ retríbuit mihi?
4 Cálicem salutáris accípiam: * et nomen Dómini invocábo.
5 Vota mea Dómino reddam coram omni pópulo ejus: * pretiósa in conspéctu Dómini mors sanctórum ejus:
6 O Dómine, quia ego servus tuus: * ego servus tuus, et fílius ancíllæ tuæ.
7 Dirupísti víncula mea: * tibi sacrificábo hóstiam laudis, et nomen Dómini invocábo.
8 Vota mea Dómino reddam in conspéctu omnis pópuli ejus: * in átriis domus Dómini, in médio tui, Jerúsalem.
I have believed, therefore have I spoken; but I have been humbled exceedingly.
I said in my excess: Every man is a liar.
What shall I render to the Lord, for all the things that he has rendered to me?
I will take the chalice of salvation; and I will call upon the name of the Lord.
I will pay my vows to the Lord before all his people: Precious in the sight of the Lord is the death of his saints.
O Lord, for I am your servant: I am your servant, and the son of your handmaid.
You have broken my bonds: I will sacrifice to you the sacrifice of praise, and I will call upon the name of the Lord.
I will pay my vows to the Lord in the sight of all his people: In the courts of the house of the Lord, in the midst of you, O Jerusalem.
And you can find notes on the individual verses of Psalm 115 starting here.