Peter describes how the sufferings of Jesus were revealed to the old testament prophets, but they didn't fully understand what they were being told.
It was the Spirit of Christ Himself that was in the prophets predicting His Own suffering to them! Indeed, there are a number of old testament texts which record the Holy Ghost revealing Christ's sufferings many centuries before Jesus was born.
The prophets did not understand who the sufferer was to be, nor what the timeline or order of events involving Messiah would be.
There was a seeming conflict between numerous prophecies of Christ's glory and absolute rule as King (e.g., Psalm 2), and Christ's suffering and dying for sin.
How could the prophets resolve these seeming contradictions? They searched and studied and prayed and sought the Lord, but the matter remained opaque to them.
This reminds us of Jesus' revelation to His disciples, that many kings and prophets had desired to see the things they saw, and hear the things they heard, but never did.
Peter goes on to state that it was revealed to the prophets that their writings were meant to minister, not in their own times, but to us who have received the full Gospel after the death and resurrection of Jesus!
That is because what they wrote without understanding it, was exactly what happened to the Lord Jesus! The predictions made by the Holy Ghost to them were fulfilled in the life and death of Jesus!
Therefore, it is not possible to claim that the Gospel was somehow made up after the fact by disappointed followers of the Savior. Paul and Peter did not make up what the dying of the Lord accomplished.
No, it had all been laid out before!