We do not know, until after the event, if we have fulfilled all we have promised.  Our claim, promise or boast is always tested by what we do.  Peter started off by boasting about his loyalty but ended by weeping about his disloyalty (Mark 14:72).  Jesus was briefing His disciples for the separation they would experience.  His mission was a solitary role; nobody else could be the substitute sacrifice for the sins of the world.  Yes, at some point in the future, Peter would also be crucified (John 21:18-19) but that would be as a martyr (the word means ‘a witness’) and not as a Saviour.
 
Peter did not understand.  Surely, they had shared all the ministry experiences together, why stop now?  Peter with James and John were privileged to see amazing things, so why not keep going with Jesus?  Jesus said that His divine assignment was unique to Him.  Indeed, they must not block His road to the cross; that was the very reason why Jesus came to earth (John 12:27).  

Peter’s arrogance in trying to protect Him had previously been rebuked by the Lord (Matthew 16:21-23), but still he did not understand.  Jesus knew what was in Peter’s heart, and what was in the future, much better than Peter ever could.  Instead, Jesus prophesied that before the next morning had dawned, Peter would deny Him three times.
 
No boasting of what we can do is from God.  Neither does it bring Him glory.  John makes this point very strongly in 1 John 2:16: “For everything in the world – the lust of the flesh, the lust of the eyes, and the pride of life – comes not from the Father but from the world.”  Every human boast will eventually fail in some way, because only what is of God will last (John 15:16).  If we claim we are the solution to God’s problem, we have not understood that we are part of the problem and God has the solution!  It is not wrong to want to help the Lord and to work for Him, but we need to be clear that He sets the agenda, and we serve as He directs so that He gets the glory.