What was Peter talking about exactly, in 1 Pet. 1:12? Whatever it was we should be excited because we are on the other side of that. Whatever it is has already been revealed since we have the complete revelation of God.
We could actually insert the entire Old Testament as the answer. Instead of that, however, we will piece together sections of it from three of the prophets to arrive at an answer.
Joel: all about locusts. Joel 2:28-32 is quoted in Acts 2. It was the prophecy that was fulfilled on the day of Pentecost in the first century. The Holy Spirit was poured out on the disciples. Joel’s prophecy was about Jesus, which of course they did not understand back then. Any reference to Zion they took as physical Jerusalem, not a spiritual kingdom as we know it.
Isaiah: multiple passages, including 32:15, 33:5-6; chapter 35; 49:8-16; 54:11-17 and 59: 16-62:12. There are references throughout these chapters to the wilderness becoming a fertile field, to Zion being filled with justice and righteousness, the wilderness blossoming profusely and fruitfulness and blessings. The new Jerusalem – us, spiritual Zion – is not just a future consideration, it is NOW. We are in it.
Zechariah: 2:8; 3:10 and 12:8-13:7. There are references to being the apple of His eye, to the Lord defending the inhabitants of Jerusalem. Again, not physical Jerusalem, but a future, spiritual Jerusalem, His church. If you read Zech 12:9, that is an explanation of what is going on in the book of Revelation. Similarly to the previous prophets, he says God will pour out His Spirit on the house of Jerusalem. In that day, living waters will pour out upon Jerusalem – just how Jesus described Himself – the living water.
These prophets were talking about the work of the Messiah and all of the blessings and privileges that would follow by being God’s child. The Messiah’s work is done. The blessings are available to everyone. Anyone can choose to be part of it and be the recipient of these blessings. Obedience to His Word is the only requirement, followed by a life of faith.