It is an odd thing that Protestants often seem unaware of just how often and how strongly Paul draws from the Deuterocanonical books, such as the book of Wisdom. Thus, when Paul talks about the Armour of the Lord, the passage to which many Protestants turn for the inspiration behind what St. Paul was saying is a short verse in Isaiah.
He put on righteousness like a breastplate,
and a helmet of salvation on his head;
he put on garments of vengeance for clothing,
and wrapped himself in fury as in a mantle.
This passage, in which God himself [signified by the pronoun] is putting on armour, is thought to be an image radically transformed by St. Paul when applied to the saints. In Ephesians 6 Paul says (note: I do think Ephesians is genuinely Pauline, as the reasons for thinking the contrary seem to be based on presuppositions I adamantly reject, including that Paul did not have, nor develop in his lifetime, an elaborate ecclesiology):
Finally, be strong in the Lord and in the strength of his power. Put on the whole armour of God, so that you may be able to stand against the wiles of the devil. For our struggle is not against enemies of blood and flesh, but against the rulers, against the authorities, against the cosmic powers of this present darkness, against the spiritual forces of evil in the heavenly places. Therefore take up the whole armour of God, so that you may be able to withstand on that evil day, and having done everything, to stand firm. Stand therefore, and fasten the belt of truth around your waist, and put on the breastplate of righteousness. As shoes for your feet put on whatever will make you ready to proclaim the gospel of peace. With all of these, take the shield of faith, with which you will be able to quench all the flaming arrows of the evil one. Take the helmet of salvation, and the sword of the Spirit, which is the word of God.
Notice, however, that this exegetical shift did not happen in a vacuum, but rather was already commonplace by the time of St. Paul. The book of Wisdom, in the Catholic Canon of the Bible (which I, being Catholic, adamantly accept), says the following:
But the righteous live for ever,
and their reward is with the Lord;
the Most High takes care of them.
Therefore they will receive a glorious crown
and a beautiful diadem from the hand of the Lord,
because with his right hand he will cover them,
and with his arm he will shield them.
The Lord will take his zeal as his whole armour,
and will arm all creation to repel his enemies;
he will put on righteousness as a breastplate,
and wear impartial justice as a helmet;
he will take holiness as an invincible shield,
and sharpen stern wrath for a sword,
and creation will join with him to fight against his frenzied foes.
This is another of those curiosities to which Catholics who know their Bibles are often privy, and to which Protestants are not; Paul is referring back to this image in the book of Wisdom. It isn’t all that different from the famous passage in Romans 1:18-21 where Paul recalls that all men know in their hearts that God exists, and suppress that truth. That truth is made evident to them by the beauty and nature of creation itself, so that creation has always made known to all men the invisible attributes of God, and thus all men are without excuse. Yet, here again, Paul is directly drawing off of the book of Wisdom:
For all people who were ignorant of God were foolish by nature;
and they were unable from the good things that are seen to know the one who exists,
nor did they recognize the artisan while paying heed to his works;
but they supposed that either fire or wind or swift air,
or the circle of the stars, or turbulent water,
or the luminaries of heaven were the gods that rule the world.
If through delight in the beauty of these things people assumed them to be gods,
let them know how much better than these is their Lord,
for the author of beauty created them.
And if people were amazed at their power and working,
let them perceive from them
how much more powerful is the one who formed them.
For from the greatness and beauty of created things
comes a corresponding perception of their Creator.
Yet these people are little to be blamed,
for perhaps they go astray
while seeking God and desiring to find him.
For while they live among his works, they keep searching,
and they trust in what they see, because the things that are seen are beautiful.
Yet again, not even they are to be excused;
for if they had the power to know so much
that they could investigate the world,
how did they fail to find sooner the Lord of these things?
Now, of course, technically Paul’s prolific use of such books as the book of Wisdom does not guarantee that it was inspired, since he also draws on Pseudepigraphal books and traditions, but it is worth taking note of just how seriously some of the Deuterocanonical books influenced his thinking. It certainly means that the argument that ‘Jesus and the Apostles never quote from that book’ looks worse than it already does. Obviously Jesus and the Apostles never quoted from a number of books, such as Esther or 1 Chronicles. They may allude to 1 Chronicles (Hebrews 5:4; Matthew 12:42 etc), but of course, once you allow for allusions, nearly all of the Deuterocanonicals are alluded to by Jesus and the Apostles. For instance, the saying ‘do unto others as you would have them do to you’ (Luke 6:31) which is a saying of Jesus, is plausibly lifted from Tobit 4:15 which says ‘and what you hate, do not do to anyone.’ That might be called a stretch, but certainly both passages are to the exact same effect, and there are clearer allusions we could draw from, such as those outlined above from Wisdom. For an index of other such references, check here.