"Imagine that your house were burning down but your whole family had escaped, and I said to you: Let me show you how much I love you! and ran into the house and died. What a tragic and pointless waste of a life, you would probably think. But now imagine that your house was on fire and one of your children was still in there, and I said to you: Let me show you how much I love you!, ran into the flames, and saved your child but perished myself. You would think: Look at how much that man loved us."
If we could save ourselves then Christ died for nothing. When we add to what Christ did, requiring this or that of ourselves or of others, we make a mockery of Christ's work on the Cross. We treat Christ's death as if it had no effect.
But Christ's death did accomplish something for those that believe in Him. Christ's death is the guarantee of our salvation and it means everything to us. And it teaches us how to go on living for him, serving him, not trying to repay him, but in an earnest attempt to please Him.
This is how we live out the gospel. Peter needed to be reminded of this way of life and may we not be too proud to admit that we need reminded too.