"'You will all fall away,' Jesus told them, 'for it is written: "I will strike the shepherd, and the sheep will be scattered." But after I have risen, I will go ahead of you into Galilee.'" NIV translation
Can you imagine how heavy the phrase "you will all fall away" must have fallen on the disciples? Jesus was referring to the fact that, in the face of great trials, all of the disciples would lose courage. They would not deny their faith in Him but they would not have the courage to stand up and be identified with Him. Jesus used the quote from Zechariah 13 to help them to see that it was all according to the plan of God.
We can look back and see that this scattering would be necessary to spread the gospel to the Gentiles and the rest of the world but they could not.
Jesus goes on to talk about the fact that He would be raised from the dead and would meet them again. He reassures them but you can imagine how they would skip over this and focus on the fact that he said they would fall away.
"Peter declared, 'Even if all fall away, I will not.' 'Truly I tell you,' Jesus answered, 'today - yes, tonight - before the rooster crows twice you yourself will disown me three times.' But Peter insisted emphatically, 'Even if I have to die with you, I will never disown you.' And all the others said the same." NIV translation
Peter immediately jumped in and pledged his unfailing support for Jesus and that might sound good but it was not true. This "great rejection" was revealed to the prophets of old through the power of God. Peter set himself up against the word of God which Jesus had quoted and, if anyone was going to be wrong, it would not be God.
This is a reminder to us, as Christians, that just because we declare something, it does not make it truth. There are many today who are taught to declare all sorts of things and to claim them as their own but we must do so with caution. As we see here, your declaration does not change the Word of God; the Word of God is designed to change you through the power of the Holy Spirit.