There is a double metaphor here in James 5:3 where money as gold and silver is personified to do two things:
- witness against wealthy money hoarders
- eat the flesh of these money hoarders
This is prophesied to occur because of the following sins:
- money is hoarded, ie not used for the benefit of others, James 5:3
- money/wages owed to workers is withheld, ie, they defrauded they workers, James 5:4
- they lived in luxury and self indulgence, James 5:5
- they were indifferent to the violence around them James 5:5
- condemned and murdered the righteous (ie innocent), that they practiced injustice for their personal benefit, James 5:6
In all these sins, the purpose is the same - to increase personal wealth at all costs, most at the cost of the poorer people or those that work for them.
Thus, it can be said that the greatest evidence against such greedy rich people in the great eternal judgement is the gold and silver they accumulated; that is, the gold and silver will "witness against them"; thus, in witnessing against the greedy rich and condemning them to eternal destruction, the gold and silver, as it were, will be the means of eating their flesh with fire!
[Special note: Despite the above, the Bible is NOT opposed to wealth; however, the Bible IS opposed to wealth obtained unjustly and in the hands of the greedy. There is no sin in being honestly rich AND generous.]
Matthew Poole summarizes this idea succinctly:
And the rust of them shall be a witness against you: by a prosopopoeia, that which properly belongs to living persons is
ascribed to dead things, as Habakkuk 2:11 Luke 19:40. It is as much as
if he had said: The rust shall be a certain evidence against you, and
which will as effectually convict you, as any living witness could do,
of your folly in putting your trust in perishing things, your
greediness in hoarding them up, your unmercifulness in not supplying
the wants of others, and your unreasonableness in denying the use of
them to yourselves, when you had rather let them lie by and perish,
than enjoy the comfort of them, or do good with them. The like
expression we have, Mark 6:11.
And shall eat your flesh; the rust (the witness of your covetousness and cruelty) which now eats your money, shall hereafter
devour yourselves, soul and body, (which he means by flesh), viz. by
procuring and kindling the wrath of God upon you, (compared to fire),
and likewise by galling your consciences with a vexatious remembrance
of your sin and folly; and so what in the judgment is a witness
against you, in hell will be a tormentor to you.
As it were fire; as if you had reserved fire in your treasure, as well as treasure in your chests.