In the end everyone leaves. There are just a few of the most loyal followers left. Jesus asks them if they also wish to leave. Peter says no we have nowhere else to go. It is not enough for God the Father to send a redeemer.
Someone has to follow that redeemer.
Someone has to believe. Somone has to take the time and the trouble to pass on that truth to others. If Peter and the others had left, we would not even know that God sent a redeemer two thousand years ago. Our belief and our response is as important to the work of God as God sending us a redeemer. If we do not believe and do not take the trouble to pass on that belief future generations will not know of Jesus and the Father.