Or at least, one of the most important keys: when the OT prophets speak to their audiences, they often speak of an ongoing redemptive process that began in their era and continues forward to our own and beyond to New Creation.
Willem VanGemeren, Interpreting the Prophetic Word, was one of my favorite textbooks in seminary, and now I get to return the favor for my OT students. Here is one gem of an observation, from p. 316:
[T]he new covenant is an eschatological reality whose fulfillment takes place in the progression of redemption, including the postexilic era, the renewal of covenant in Jesus Christ, and the present church age.
He then cites Calvin at length:
Hence the Prophet here intimates that God’s favor would be certain, because he would not only give leisure to the Jews, when they returned, to plant vines, but would also cause them to enjoy the fruit in peace and quietness. . . . He extends God’s favour to the country and the villages, as though he had said, that the land would be filled with inhabitants, not only as to the fortified towns, but as to the fields…Now, were one to ask, when was this fulfilled? We must bear in mind what has been said elsewhere,—that the Prophets,…included the whole Kingdom of Christ from the beginning to the end. And in this our divines go astray, so that by confining these promises to some particular time, they are compelled to fly to allegories; and thus they wrest, and even pervert all the prophecies. But the Prophets, as it has been said, include the whole progress of Christ’s Kingdom when they speak of the future redemption of the people. The people began to do well when they returned to their own country; . . . It was, therefore, necessary for them to look for the coming of Christ. We now taste of these benefits of God . . . We hence see that these prophecies are not accomplished in one day, or in one year, no, not even in one age, but ought to be understood as referring to the beginning and the end of Christ’s Kingdom.
Calvin is commenting on Jer 31:5, 24 (emphasis is WVG’s). We can also cite similar comments from Calvin’s commentary on Isaiah 52:8.
When he restored the Jews to liberty, and employed the ministry of Zerubbabel, Erza, and Nehemiah, these things were fulfilled. Yet at the same time they ought to be continued down to the coming of Christ, by which the church was gathered out of all parts of the world. But we ought also to go forward to Christ’s last coming, by which all things shall be perfectly restored.