"καὶ στρατείας κλῆσις" - Cyril, Procatechesis, §1 In Procatechesis, §1, Cyril was talking about events that had occurred before a Christian was baptized. 

He wrote,


  Ὀνοματογραφία τέως ὑμῖν γέγονε, καὶ στρατείας κλῆσις· καὶ νυμφαγωγίας λαμπάδες, καὶ οὐρανίου πολιτείας ἐπιθυμία, καὶ πρόθεσις ἀγαθὴ, καὶ ἐλπὶς ἐπακολουθοῦσα·


My translation:


  Hereto, there has been an inscription of your names, and a στρατείας κλῆσις, and torches of a bridal procession, and a desire of a heavenly citizenship, and a good purpose, and an attending hope. 


My question is about the στρατείας κλῆσις. The Greek word κλῆσις means "call" (i.e., calling; cp. Rom. 11:29). Philip Schaff described the phrase "as a call to military service."

What event in the life of a Christian does this describe? When does this call to "military service" occur? Do any of the NT authors describe it?
One example of military items in the service of the Lord occurs in the Epistle to the Ephesians:

6 10   Finally, be strong in the Lord and in the strength of his might. 11 Put on the whole armor of God, that you may be able to stand against the wiles of the devil. 12 For we are not contending against flesh and blood, but against the principalities, against the powers, against the world rulers of this present darkness, against the spiritual hosts of wickedness in the heavenly places. 13 Therefore take the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand. 14 Stand therefore, having girded your loins with truth, and having put on the breastplate of righteousness, 15 and having shod your feet with the equipment of the gospel of peace; 16 besides all these, taking the shield of faith, with which you can quench all the flaming darts of the evil one. 17 And take the helmet of salvation, and the sword of the Spirit, which is the word of God.  [RSV-2CE]

The helmet of salvation is found in the traditional prayers a priest makes while vesting for Mass, specifically, putting on the amice:

Impone, Domine, capiti meo galeam salutis, ad expugnandos diabolicos incursus.
Lord, set the helmet of salvation on my head to fend off all the assaults of the devil.

The metaphor is carried through to modern times too. The Church of England baptism service introduced in 1980 had:

Name,    I sign you with sign of the cross.
Do not be ashamed to confess the faith of Christ crucified.
Fight valiantly under the banner of Christ against sin, the world, and the devil,
and continue his faithful soldier and servant to the end of your life.

The Church is often described as the Church militant, expectant and triumphant, explained by Wikipedia with reference to the above passage from Ephesians:

In Christian theology, the Christian Church, or Church Universal, is traditionally divided into:

the Church Militant (Ecclesia Militans), comprising Christians on earth who are living; Christian militia, who struggle against sin, the devil and &quot;..the rulers of the darkness of this world, against spiritual wickedness in high places&quot; (Ephesians 6:12).
the Church Triumphant (Ecclesia Triumphans), comprising those who are in Heaven, and
the Church Suffering, a.k.a. Church Padecent or Church Penitent (Ecclesia Penitens) or Church Expectant (Ecclesia Expectans), which in Catholic theology comprises those Christians presently in Purgatory.