Notice in v.70 how Jesus responds to Peter’s statement from v.69.  He does not deny anything Peter said since all of it is 100% true, but he does seek to ground it in his own sovereign work—Is it not the case that I myself chose you, the 12? Jesus’ point here can be captured by these words: Is not the main difference between you and them my intentional and specific selection of you? 

In saying this, Jesus is combatting the common human tendency of placing confidence in something we have done for salvation.  Think about the way we share our testimony, specifically with regards to what gets the emphasis.  Do we articulate salvation in terms of when I accepted Jesus into my heart, when I walked forward at a camp, when I prayed a prayer after listening to a message online?  Those may be the circumstances involved in our salvation, but those actions should never be thought of as the cause or ground of it.  Sure, we must believe in Jesus to be saved, but we must not have faith in our faith

Instead, all our confidence and boasting must be in God’s saving work.