According to the apparatus in the Nestle-Aland Greek text, there are no textual variants in any of the available manuscripts that have something other than ... κόσμου ... κόσμου ..., so this is not a case of the Vulgate relying on a unique Greek manuscript.
I lay out my case below, but I believe we are seeing a case where Jerome took advantage of Latin words at his disposal that could distinguish shades of meaning in a particular Greek word.
Possible meanings of the Greek
According to Bauer's Greek-English Lexicon of the New Testament and Early Christian Literature, the Greek word κόσμος (κόσμου is the genitive singular masculine of κόσμος) has eight (8) distinct meanings:
- adornment, adorning
- in philosophical usage the world as the sum total of everything here and now, *the (orderly) universe
- the world as the sum total of all beings above the level of animals
- the world as the earth, the planet upon which we live
- the world as mankind
- the world as the scene of earthly joys, possessions, cares, sufferings
- the world, and every thing that belongs to it, appears as that which is hostile to God, i.e. lost in sin, wholly at odds w. anything
divine, ruined and depraved
- totality, sum total
According to Bauer, the meaning understood for the first occurrence of κόσμος in James 4:4 is that corresponding to #7 above, but it's not clear to me which meaning is imputed to the second occurrence (the entry has "Js 4:4a; cf. b").
Possible meanings of the Latin
According to Harden's Dictionary of the Vulgate New Testament, the noun mundus means "world", "universe", or "the world". According to the same source, saeculum - from which we get the English word "secular" - has two definitions: (1) "age" or "generation"; or (2) "world". There is probably a better Latin lexicon that delineates meanings to the detail that Bauer dissects the Greek, but I don't have access to it.
Knowing only that some foreign word can mean "world" in English is not completely helpful, since, according to the Oxford English Dictionary, the word "world" has 20 distinct meanings.
Possible explanation for two different words in Latin
Assuming that the Vulgate translator(s) - probably originating with Jerome - did not make some kind of error (which I don't believe they did), the only explanation I can think of is that the translator wanted to capture the distinct meaning of the Greek word in its particular context.
The commentary of Bede here might be particularly helpful. Bede, sometimes referred to as Venerable Bede, was an English Benedictine monk who lived between 675 and 735 AD, and was a prolific historian and commentator on Scripture. His source was Jerome's Latin text and his commentaries were written in Latin, even though he was also perfectly schooled in Greek. In his commentary on the James 4:4 he writes:
Adulteri, nescitis quia amicitia hujus mundi inimica est Dei? (Adulterers, do you not know that the friendship of this world is an
enemy of God?)
He properly calls adulterers those whom he rebukes for having
abandoned the love of heavenly wisdom and turned instead to the
clutches of worldly friendship, whom he saw despising the Creator to
serve mammon. Indeed he had said above concerning the open enemies of
God, Do not the rich oppress you by their power and drag you to
judgments? Do they not blaspheme the good name that has been called
down upon you? (2:6-7).
But that you might not consider enemies of God only those who openly blaspheme him, who persecute the saints for their faith in him, and condemn them by unjust judgments, he shows that they are also enemies of God who after faith and confession of his name become slaves to the delights and love of the world, who are faithful in name only and prefer earthly to heavenly things. He also insists more urgently upon this in the following verse, appending,
Quicumque ergo voluerit amicus esse sæculi hujus, inimicus Dei constituitur (*Whoever, therefore, wishes to be a friend of this
world is an enemy of God.
Therefore, all lovers of the world, all seekers after trifles, are
enemies of God; all belong to those of whom it is said, Look, how
your enemies, O Lord, will perish (Psalm 91:10 LXX) They may enter
the churches, they may not enter the churches, they are enemies of
God. For a time they are able to flourish as grass, but when the heat
of judgment appears they will perish and the loveliness of their
countenance will vanish (1:10-11)1
Thus, I think, Bede gives us to understand why κόσμος is first translated as mundus, and then as saeculus in the second instance in the same verse:
- In the first occurrence, Jerome (we suppose) understood that κόσμος was being used in the sense that Bauer defines in #7 of his lexicon entry. Mundus was understood at the time to be the Latin word closest to this particular meaning.
- In the second sense, it would seem that κόσμος was understood in the more mundane sense - the seeking after trifles, as Bede calls it - the sense Bauer tries to convey in #6: κόσμος "as the scene of earthly joys, possessions, cares". Similarly, saeculus was understood to be the Latin word closest to this particular meaning.
Is this consistent, though, with how a Greek Church Father would understand the verse - reading in Greek and commenting in Greek?
I believe so.
Symeon the New Theologian, a latter Greek Father who lived in the late 10th/early 11th century, seems to interpret the verse in a fashion very similar to that of Bede:
By these words he shows us it is not only he who commits sin who is separated from God and becomes His enemy, but he who loves it and covets something, or has an attachment in his heart to anything that is on earth, constitutes friendship with the world ...2
1. Commentary on the Seven Catholic Epistles (tr. from Latin; Cistercian Press, 1985), pp. 48-49
2. Discourse V, "On Penitence" XVIII; tr. from Greek in Dmitry Royster, The Epistle of Saint James: A Commentary (St. Vladimir's Seminary Press, 2010), p. 95