At Westminster Seminary California in OT 702 Prophetical Books, Dr. Bryan Estelle taught us about ‘prophetic idiom.’ Prophetic idiom describes the phenomena of the Old Testament prophets using Old Covenant language in order to speak of future New Covenant realities.
Here is a passage from Calvin wherein he teaches this same basic notion of ‘prophetic idiom.’
Calvin commenting on Haggai 2:8, “The silver is mine, the gold is mine, saith the Lord of hosts.”
“But why does the Prophet mention gold and silver? He did this in conformity with what was usual and common; for whenever the Prophets speak of the kingdom of Christ, they delineate or describe its splendour in figurative terms, suitableto [sic.] their own age. When Isaiah foretells the restoration of the Church, he declares that the Church would be all gold and silver, and whatever glittered with precious stones; and in ch. lx. he especially sets forth the magnificence of the Temple, as though nations from all parts were to bring for sacrifice all their precious things. But Isaiah speaks figuratively, as all the other Prophets do. So then what we read of gold and of silver ought to be so explained as to be applied mystically to the kingdom of Christ…” (Calvin, Commentaries on Haggai, pg. 363).
A Pilgrim's Theology 