A witness against you. He confirms the explanation I have already given. For God has not appointed gold for rust, nor garments for moths; but, on the contrary, be has designed them as aids and helps to human life. Therefore, even spending without benefit is a witness of inhumanity. The rusting of gold and silver will be, as it were, the occasion of inflaming the wrath of God, so that it will, like fire, consume them. Ye have heaped treasure together: These words may also admit of two explanations: -- that the rich, as they would always live, are never satisfied, but weary themselves in heaping together what may be sufficient to the end of the world, -- or, that they heap together the wrath and curse of God for the last day; and this second view I embrace. [1]

Footnotes

1 - By "last days" are commonly meant the days of the gospel. The day of judgment is often called by John, in his Gospel, "the last day;" and the same seems to be called here "the last days." The reference made by some, to the destruction of Jerusalem, has nothing in the passage to favor it. To "heap treasure," or to lay up a store, has an evident reference to the day of judgment, as Paul makes use of the same expression in Romans 2:5, only he adds "wrath" to it, which is also added here by the Vulg. The whole verse is conminatory, and in this sentence the rich are reminded of the issue, the final issue of their conduct. The character of the store is to be learnt from the preceding part of the verse. In treasuring dishonest wealth, they were treasuring wrath for themselves.

Your gold and silver is cankered - That is, that you have heaped together, by injustice and fraud, a large amount, and have kept it from those to whom it is due, James 5:4, until it has become corroded. The word rendered is "cankered" (κατίωται katiōtai,) does not occur elsewhere in the New Testament. It properly means "to cause to rust; to rust out" (Passow); "to be corroded with rust" (Robinson); to be spotted with rust. It is true that gold and silver do not properly rust, or become oxidized, and that they will not be corroded like iron and steel; but by being kept long in a damp place they will contract a dark color, resembling rust in appearance. This seems to be the idea in the mind of the apostle. He speaks of gold and silver as they appear after having been long laid up without use; and undoubtedly the word which he uses here is one which would to an ancient have expressed that idea, as well as the mere literal idea of the rusting or oxidizing of metals. There is no reason to suppose that the word was then used in the strict chemical sense of rusting, for there is no reason to suppose that the nature of oxidization was then fully understood.
And the rust of them - Another word is used here - ἰὸς ios. This properly denotes something sent out or emitted, (from ἕημι hēmi), and is applied to a missile weapon, as an arrow; to poison, as emitted from the tooth of a serpent; and to rust, as it seems to be emitted from metals. The word refers to the dark discoloration which appears on gold and silver, when they have remained long without use.
Shall be a witness against you - That is, the rust or discoloration shall bear testimony against you that the money is not used as it should be, either in paying those to whom it is due, or in doing good to others. Among the ancients, the gold and silver which anyone possessed was laid up in some secret and safe place. Compare the notes at Isaiah 45:3. There were no banks then in which money might be deposited; there were few ways of investing money so as to produce regular interest; there were no corporations to employ money in joint operations; and it was not very common to invest money in the purchase of real estate, and stocks and mortgages were little known.
And shall eat your flesh as it were fire - This cannot be taken literally. It must mean that the effect would be as if it should corrode or consume their very flesh; that is, the fact of their laying up treasures would be followed by painful consequences. The thought is very striking, and the language in which it is conveyed is singularly bold and energetic. The effect of thus heaping up treasure will be as corroding as fire in the flesh. The reference is to the punishment which God would bring on them for their avarice and in-justice - effects that will come on all now for the same offences.
Ye have heaped treasure together for the last days - The day of judgment; the closing scenes of this world. You have been heaping up treasure; but it will be treasure of a different kind from what you have supposed. It is treasure not laid up for ostentation, or luxury, or use in future life, but treasure the true worth of which will be seen at the judgment-day. So Paul speaks of "treasuring up wrath against the day of wrath, and revelation of the righteous judgment of God," Romans 2:5. There are many who suppose they are accumulating property that may be of use to them, or that may secure them the reputation of possessing great wealth, who are in fact accumulating a most fearful treasure against the day of final retribution. Every man who is rich should examine himself closely to see whether there is anything in the manner in which he has gained his property, or in which he now holds it, that will expose him to the wrath of God in the last day. That on which he so much prides himself may yet bring down on him the vengeance of heaven; and in the day of judgment he may curse his own madness and folly in wasting his probation in efforts to amass property.

Your gold and silver is cankered - Instead of helping the poor, and thus honoring God with your substance, ye have, through the principle of covetousness, kept all to yourselves.
The rust of them shall be a witness against you - Your putrefied stores, your moth-eaten garments, and your tarnished coin, are so many proofs that it was not for want of property that you assisted not the poor, but through a principle of avarice; loving money, not for the sake of what it could procure, but for its own sake, which is the genuine principle of the miser. This was the very character given to this people by our Lord himself; he called them φιλαργυροι, lovers of money. Against this despicable and abominable disposition, the whole of the 12th chapter of St. Luke is levelled; but it was their easily besetting sin, and is so to the present day.
Shall eat your flesh as it were fire - This is a very bold and sublime figure. He represents the rust of their coin as becoming a canker that should produce gangrenes and phagedenous ulcers in their flesh, till it should be eaten away from their bones.
Ye have heaped treasure together - This verse is variously pointed. The word ὡς, like as, in the preceding clause, is left out by the Syriac, and some others; and πυρ, fire, is added here from that clause; so that the whole verse reads thus: "Your gold and your silver is cankered; and the rust of them shall be a witness against you, and shall consume your flesh. Ye have treasured up Fire against the last days." This is a bold and fine image: instead of the treasures of corn, wine, and oil, rich stuffs, with silver and gold, which ye have been laying up, ye shall find a treasure, a magazine of fire, that shall burn up your city, and consume even your temple. This was literally true; and these solemn denunciations of Divine wrath were most completely fulfilled. See the notes on Matthew 24 (note), where all the circumstances of this tremendous and final destruction are particularly noted.
By the last days we are not to understand the day of judgment, but the last days of the Jewish commonwealth, which were not long distant from the date of this epistle, whether we follow the earlier or later computation, of which enough has been spoken in the preface.

Your gold and silver is cankered,.... Or grown rusty like iron, by lying long without use; this is not easily and quickly done, but in length of time gold and silver will change, and contract a rustiness; and so this conveys the same idea of hoarding up riches and laying up money, without making use of it in trade, for the support of the poor, and without distributing it to their necessities:
and the rust of them shall be a witness against you: at the day of judgment; which will be a proof that they have not been employed to such services, and for such usefulness, for which they were designed and given.
And shall eat your flesh as it were fire; that is, a remembrance of this, a sense of it impressed upon them, shall be like fire in their bones; shall distress their minds, gnaw their consciences, and be in them the worm that never dies, and the fire that shall never be quenched:
ye have heaped treasure together for the last days; either for many years, as the fool in the Gospel, for the times of old age, the last days of men, for fear they should then want; or for the last days of the world, or of time, as if they thought they should live for ever: the Vulgate Latin version reads, "ye have treasured up wrath for yourselves in the last days"; instead of riches, as they imagined; and that by their covetousness and wickedness, by a wicked disuse of their riches, and an unrighteous detention of them; but this supplement seems to be taken from Romans 2:5 though the sense is confirmed by some copies which connect the phrase, "as it were fire", in the preceding clause, with this, "ye have treasured up as it were fire"; and the Syriac version renders it, "ye have treasured up fire"; the fire of divine wrath; this is the fruit of treasuring up riches in an ill way, and without making a proper use of them.

is cankered--"rusted through" [ALFORD].
rust . . . witness against you--in the day of judgment; namely, that your riches were of no profit to any, lying unemployed and so contracting rust.
shall eat your flesh--The rust which once ate your riches, shall then gnaw your conscience, accompanied with punishment which shall prey upon your bodies for ever.
as . . . fire--not with the slow process of rusting, but with the swiftness of consuming fire.
for the last days--Ye have heaped together, not treasures as ye suppose (compare Luke 12:19), but wrath against the last days, namely, the coming judgment of the Lord. ALFORD translates more literally, "In these last days (before the coming judgment) ye laid up (worldly) treasure" to no profit, instead of repenting and seeking salvation (see on James 5:5).

The canker of them - Your perishing stores and motheaten garments. Will be a testimony against you - Of your having buried those talents in the earth, instead of improving them according to your Lord's will. And will eat your flesh as fire - Will occasion you as great torment as if fire were consuming your flesh. Ye have laid up treasure in the last days - When it is too late; when you have no time to enjoy them.