5:3 Your gold and silver is cankered. Rusted from disuse. These metals do not literally rust, but do tarnish from long disuse. The idea is that they show they have been hoarded, not used.
And the rust of them shall be a witness against you. The tarnish shows that you have hoarded instead of using.
And shall eat your flesh as it were fire. They shall punish you, as though heated by fire and eating into your flesh. This hoarded wealth will curse the possessor.
Ye have heaped up treasure. Instead of laying up treasure in heaven you have continued to pile up earthly treasure to the last, a matter of extreme folly.
For the last days. What James then thought of as the last days, the end of his nation, country and Jerusalem, was close at hand.
5:3 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.
5:1-6 Public troubles are most grievous to those who live in pleasure, and are secure and sensual, though all ranks suffer deeply at such times. All idolized treasures will soon perish, except as they will rise up in judgment against their possessors. Take heed of defrauding and oppressing; and avoid the very appearance of it. God does not forbid us to use lawful pleasures; but to live in pleasure, especially sinful pleasure, is a provoking sin. Is it no harm for people to unfit themselves for minding the concerns of their souls, by indulging bodily appetites? The just may be condemned and killed; but when such suffer by oppressors, this is marked by God. Above all their other crimes, the Jews had condemned and crucified that Just One who had come among them, even Jesus Christ the righteous.