What timeline was the Old Testament looking forward to?
The Old Testament looked forward to the timeline in which Christ would come, suffer, die, and be raised from the dead. After that, the wrath of God would fall, and then the Kingdom would be set up. This was the prophetic timeline Peter understood.
Les explains that the Old Testament was looking forward only to this top line: Christ would come, suffer, die, be raised from the dead, then the wrath of God would fall, and then the Kingdom would be set up. He contrasts that with the church age, noting that the Gospels said not a word about God turning to the Gentiles and calling out of them a body of Christ, of Jew and Gentile. Peter, therefore, had no concept of anything other than this Old Testament timeline. At Pentecost, when accused of drunkenness, Peter responded in Acts 2:15 that it was only the third hour, and he identified what was happening as the fulfillment of prophecy.