"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.