Mark 14:31

But he spake exeseding vehemently ( ἐκπερισσῶς ἐλάλει), If I must die with thee ( ἐάν με δέρ), I will not deny thee. The right reading ( ἐλάλει, imperfect) implies that he kept asserting over and over again.

He was, no doubt, sincere in all this, but he had vet to learn his own weakness. St. Hilary says on this, "Peter was so carried away by the fervor of his zeal and love for Christ, that he regarded neither the weakness of his own flesh nor the truth of his Master's word."