With Christ there is plenteous redemption: Psalm 129 (Gradual Psalm No 11)

As well as being one of the Gradual Psalms, Psalm 129 is also the first psalm of Tuesday Vespers, one of the seven penitential psalms; and is used in the Office of the Dead at Vespers.  It is a traditional preparatory prayer for Mass; and it carries an indulgence if said for those in purgatory.

Psalm 129: De Profundis

Vulgate	Douay-Rheims
Canticum graduum.	Canticum graduum.
De profúndis clamávi ad te, Dómine: * Dómine, exáudi vocem meam :	Out of the depths I have cried to you, O Lord:
2  Fiant aures tuæ intendéntes: * in vocem deprecatiónis meæ.	2 Lord, hear my voice. Let your ears be attentive to the voice of my supplication.
3  Si iniquitátes observáveris, Dómine: * Dómine, quis sustinébit?	3 If you, O Lord, will mark iniquities: Lord, who shall stand it.
4  Quia apud te propitiátio est: * et propter legem tuam sustínui te, Dómine.	4 For with you there is merciful forgiveness: and by reason of your law, I have waited for you, O Lord.
5  Sustinuit ánima mea in verbo ejus: * sperávit ánima mea in Dómino.	My soul has relied on his word: 5 My soul has hoped in the Lord.
6  A custódia matutína usque ad noctem: * speret Israël in Dómino.	6 From the morning watch even until night, let Israel hope in the Lord.
7  Quia apud Dóminum misericórdia: * et copiósa apud eum redémptio.	7 Because with the Lord there is mercy: and with him plentiful redemption.
8  Et ipse rédimet Israël: * ex ómnibus iniquitátibus ejus.	8 And he shall redeem Israel from all his iniquities

Historical context

The psalm is almost certainly Davidic in origin, as 2 Chronicles 6:36-42, which is part of a prayer of King Solomon, alludes to and explains this psalm, and explicitly mentions Solomon's father, King David in this context.

Here are the verses in question from Chronicles:

"And if they sin against you (for there is no man that sins not) and you be angry with them, and deliver them up to their enemies, and they lead them away captive to a land either afar off, or near at hand, and if they be converted in their heart in the land to which they were led captive, and do penance, and pray to you in the land of their captivity saying: We have sinned, we have done wickedly, we have dealt unjustly: And return to you with all their heart, and with all their soul, in the land of their captivity, to which they were led away, and adore you towards the way of their own land which you gave their fathers, and of the city, which you have chosen, and the house which I have built to your name: Then hear from heaven, that is, from your firm dwelling place, their prayers, and do judgment, and forgive your people, although they have sinned: For you are my God: let your eyes, I beseech you, be open, and let your ears be attentive to the prayer, that is made in this place. Now therefore arise, O Lord God, into your resting place, you and the ark of your strength: let your priests, O Lord God, put on salvation, and your saints rejoice in good things. O Lord God, turn not away the face of your anointed: remember the mercies of David your servant."

Christ reconciles us, reopening the way to heaven

The focus of this psalm is Christ's work of reconciliation: we have now surely reached the harrowing of hell and reopening of the way to heaven in Christ's life recounted in the psalms of the steps.

In this light, Pope Benedict XVI suggested that:

"the text is first and foremost a hymn to divine mercy and to the reconciliation between the sinner and the Lord, a God who is just but always prepared to show himself "a merciful and gracious God, slow to anger and rich in kindness and fidelity, continuing his kindness for a thousand generations, and forgiving wickedness and crime and sin" (Ex 34: 6-7)."

In terms of our own ascent in imitation of him, one can perhaps use it to meditate on incidents in the Gospel such as Jesus' repeated forgiveness of sins of those he healed through his miracles; the tearful repentance of Mary Magdalene; the story of the woman caught in adultery and more.

Competing textual traditions?

It is worth noting that this is one of those psalms where the Septuagint Greek (and thus Vulgate) and the (late medieval) Hebrew Masoretic Text are in places very different, in ways impossible to reconcile by looking for alternative readings of the Hebrew.  

In particular, from verse 4 onwards, the Hebrew puts much more emphasis on fear of God, and omits two references to the hope of the Christ’s redemption.  This may well be the result of early rabbinical reaction to Christianity, and in fact the text is so corrupt that in places even protestant bibles that usually prefer the Hebrew have adopted the Vulgate tradition.

Liturgical and Scriptural uses of the psalm

NT references	Rom 3:24 (v7); Mt 1:21, Tit 2:14, Rev 1:5 (v8)
RB cursus	Tuesday Vespers+AN 2116 (1-2); 4990 (4)
Monastic feasts etc	Gradual Psalms Penitential Psalms Vespers of Christmas and its octave (alternating days); 1 Vespers of Sacred Heart; Office of the Dead V AN 5094(4-5), 1446 (7)
Roman pre 1911	Wednesday Vespers
Responsories	Epiphanytide Sat v2 (6495); 7143 (alt verse for Sept 3&4)
Roman post 1911	1911-62: Wednesday Vespers
Mass propers (EF)	Septuagesima Sunday: TR (1-4) PP22 IN 1-4 PP 23: AL (102); OF (1-2)

You can find notes on the individual verses of the psalm in this series, in the context of Vespers:

Introduction to Psalm 129
Ps 129 v 1-2
Ps 129 v3-5a
Ps 129 v5b-6
Ps 129 v7-8