Onward Christian soldiers

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"Grant, O Lord, that we may begin with holy fasting this campaign of Christian service, so that, as we take up battle against spiritual evils, we may be armed with weapons of self-restraint."

     Collect for Ash Wednesday, Roman missal, translation of 2010.

Concede nobis, Domine, praesidia militiae christianae sanctis inchoare ieiuniis, ut, contra spiritales nequitias pugnaturi, continentiae muniamur auxiliis.

Critical 1956 edition ed. Mohlberg:

Concede nobis, domine, praesidia mili[ti]ae christianae sanctis incoare ieiuniis, ut contra spiritales nequitias pugnatori continentiae muniamur auxiliis:  per.

     Collect for Ash Wednesday, Missale Romanum.  =Corpus orationum no. 673 (vol. 1, pp. 326-327), which lists a few minor variants (most importantly salutaris for continentiae in Bergomensis (second half of the 9th century)) =Bruylants no. 117 (vol. 2, p. 39).  This one is present in the Veronese or "Leonine" sacramentary (Verona, Biblioteca capitolare LXXXV (80)), which was written in the 2nd half of the 6th century, but contains prayers that can be traced back into the 1st half of the 5th.  This collect is thought by some to be one of those.

Perisho (recognizing that it is impossible to get all of the harmonics of the Latin into a single, word-for-word translation; cf. e.g. nequitia in the Vulgate of Eph 6:12 & 1 Cor 5:8):

Grant to us, O Lord, with holy fasts to found [(inchoare)] garrisons [(praesidia)] of Christian valor [(militiae, also warfare)], that, contending in battle [(pugnaturi)] against spiritual laxities [(nequitias, also evils, wickedness)], we may be reinforced [(muniamur)] by auxiliaries [(auxiliis)] of self-control [(continentiae)].

Perisho again in October of 2018, treating pugnatori as the noun (not, alas, adjective) in the dative singular:

Grant to us, O Lord, to begin [to throw up] the defense-works of th[is] Christian campaign with holy fasts, that we, as combatant[s], may be fortified against the spiritual [powers] of wickedness by auxiliaries of continence.

Fr. Z:

Grant us, O Lord, to commence the defenses of the Christian field campaign by means of holy fasts, so that, we who are about to do battle against spiritual negligences, may be fortified by the support of continence.

First things:

"Grant us, Lord, to begin our active service in the camp of the Christian army with holy fasts, so that we who are about to do battle against spiritual wickedness may be strengthened by the help of continence."

Roman missal as translated before 2010, reproduced by Fr. Z, and included here for laughs:

Father in Heaven, Protect us in our struggle against evil.  As we begin the discipline of Lent, make this season holy by our self-denial.

     This, by the way, is the closest that—employing the Latin-word and Scripture (Eph 6:11-12 ("accipite armaturam Dei", "tenebrarum," "induite vos arma Dei"), Rom 8:37 ("superamus propter eum qui dilexit nos")) indices of the 1956 Mohlberg edition of 1956, the online Vatican (?) concordance, and the 1896 edition ed. Feltoe—I've been able to come to the following prayer.  Apparently somebody took some liberties in claiming this for the "Leonine" (or Veronese) Sacramentary:

Grant us, O Lord, so to enter on the service of our Christian warfare, that, putting on the whole armour of God, we may endure hardness and fight against the spiritual powers of darkness, and be more than conquerors through him that loved us, Jesus Christ our Lord.