Gratia tua ne nos, quaesumus, Domine, derelinquat,
quae et sacrae nos deditos faciat servituti,
et tuam nobis opem semper acquirat.
If this prayer seems "odd" to you for some reason, you are on the right track. While there is nothing at all wrong with the prayer, it seems out of place… because in a sense it is. In the ancient Gelasian Sacramentary this was a Post Communion for the Wednesday of the 2nd Week of Lent. In the 1962 Missale Romanum and its previous editions, this was the Post Communion of Thursday in the same week. So, the redactors of the Novus Ordo extracted this from its place and inserted it into its present spot. Why? There must have been a thematic reason.
The verb derelinquo is "to forsake wholly, to abandon, desert" and also "to leave behind" (even in the sense of inheritance). Someone who is a "derelict", is really someone abandoned, utterly forsaken.
Acquiro (or ad-quiro) is "to add to, to get or acquire".
Deditos is probably the adjectival form deditus, a, um "given up to, addicted, devoted to something; eager, assiduous, diligent", derived from dedo.
It is entirely possible that the phrase "gratia tua" refers not just literally to the freely given gift which we call "grace", but also is a courtly form of address for God, "Your Grace". This use of substantive or adjective with tua an address is fairly common of Latin in the age whence this prayer comes.
We have here in quae + subjunctive faciat… acquirat a characteristic result clause. These are sometimes a little hard to get into smooth English without making the original Latin structure nearly disappear. That’s okay, of course, in making a smooth liturgically useful prayer. However, in these WDTPRS articles it is our objective to stick closely to the original so that you can see for yourselves what is really going on inside the Latin. So, put on your archaic sounding English caps for a moment, and ready yourselves for an older application of "might".
Let not Your Grace abandon us, O Lord, we beg,
which might make us eager for Your holy service,
and always acquire for us Your assistance.
As you can see, "might" here, does not have the force of "maybe", but rather coveys an auxillary force of probability or purpose.
PUT ANOTHER WAY
O Lord we beg You, let not Your grace forsake us,
which, in our having it, shall result in us being made eager for the sacred service You determine
and shall always obtain for us Your support.
OR YET ANOTHER WAY
O Lord, we beg You, let not Your grace desert us,
for we need it in order to be made eager for Your holy service,
and it must obtain for us Your succor.
A MOCKING LAME-DUCK ICEL VERSION
you are with us.
Help us serve you always.
As we think about this prayer, remember that originally it was recited by the priest after Communion, rather than at the beginning of Mass. So, we are praying herewith that the graces and effects of the Communion just received would endure. Fairly soon after this prayer we receive the final blessing, wait for the last Gospel and (for a few decades at least) the Leonine prayers after Mass. Then after a quiet moment of thanksgiving out of church we would go to our work, whatever that might be.
Perhaps in order to get our mind around this prayer today we can think in terms of the rhetorical device called hysteron proteron. This Greek term, literally, "the latter, the former" refers to a reversal of time or sequence to create an effect. For example, we do this everyday when we "put on our shoes and socks". I normally put on my socks and then my shoes. A more serious example is found in Dante’s Commedia where in order to covey something of the super speed of their ascent to the heaven of the moon the Poet says that it took no longer than it takes an arrow "to strike, fly, and leave the bow" (Par 2.23-6). The word "strike" comes first, to emphasize its importance, even though chronologically it occurs after the shot and flight of the arrow. In Shakespeare you find this all the time. For example in Anthony and Cleopatra we hear "Th’ Antoniad, the Egyptian admiral, / With all their sixty, fly and turn the rudder" (III, x, 1-2). Clearly you have to turn the rudder before you can flee.
Give this some consideration now for your participation at Mass. If you go into church with the effects and the results firmly in mind before Mass starts, your active participation might take on a different quality. Reception of Communion in the state of grace is the most perfect kind of "active participation" at Mass. Thus, think of the Communion which is to come at the Collect so that you may be more recollected at Communion. See every moment and action, every word and gesture, of Mass in light of Communion. Do the same even when you are NOT able to receive!