We believe that we really do know ourselves. Peter believed that too. But when Peter was tempted he didn't have the strength to manifest the words that he had promised the Lord. 

Matthew 26:31-35 King James Version (KJV)

31 Then saith Jesus unto them, All ye shall be offended because of me this night: for it is written, I will smite the shepherd, and the sheep of the flock shall be scattered abroad.

32 But after I am risen again, I will go before you into Galilee.

33 Peter answered and said unto him, Though all men shall be offended because of thee, yet will I never be offended.

34 Jesus said unto him, Verily I say unto thee, That this night, before the cock crow, thou shalt deny me thrice.

35 Peter said unto him, Though I should die with thee, yet will I not deny thee. Likewise also said all the disciples.