When Peter denied Christ (3 times) at that point you believe that he was denied and lost his salvation. If you believe that, when did Peter repent for his denials and get his salvation back? He certainly could not have preached and filled with the Spirit at Pentecost and lead 1,000's to salvation in Christ if he were an unsaved man who lost his salvation. Don't presume that it must of happened, show us somewhere in the scriptures that Peter repented and got 'resaved'. It could not have been associated with / John 14:14-19 because Christ was dealing with Peter's heart in relationship to loving Him and feeding His flock and not salvation. If you can't show any evidence for what you believe about Peter then you have no grounds for what you believe.
Yes it is a good point you make. But unfortunately for your case Peter's repentance is recorded in two gospels in the bible, and occurs almost immediately after his denial of Christ:
Luk 22:61 And the Lord turned and looked on Peter. And Peter remembered the word of the Lord, how He had said to him, Before the cock crows, you shall deny Me three times.
Luk 22:62 And Peter went out and wept bitterly.
Mat 26:75 And Peter remembered the word of Jesus, Who said to him, Before the cock crows, you shall deny Me three times. And he went out and wept bitterly.
(On a side note, interestingly Peter was not only a deniar of Christ but a profane man at that point:
"Mat 26:74 Then he began to curse and to swear"
A few good bible commentaries on this verse that confirms this was Peters repentance:
PNT:
He went out and wept bitterly. The look of Christ broke his heart. As the cock crew, his own confident assertions and the word of the Lord, "Before the cock crow twice (before the second cock crowing) thou shalt thrice deny me," rushed upon him. He rushed out into the darkness of the night to weep. Judas sinned, betrayed and sold the Lord from covetousness. Afterward he was sorry, but it was the sorrow of this world that worketh death. It was remorse, not repentance, and he went and hanged himself. Peter's repentance was attested (1) by the bitterness of his tears; (2) by his humble submission to his Lord's subsequent rebuke (Joh_21:15-17); (3) by his subsequent courage in confessing Christ in the face of threatening danger (Act_4:8-12, Act_4:19).
Gill:
and wept bitterly; being thoroughly sensible what an evil and bitter thing the sin was, he had been guilty of: his repentance sprung from Christ's looking upon him, and from his looking to Jesus, and was truly evangelical: it was a sorrow after a godly sort, and was increased by the discoveries of Christ's love unto him; and was attended with faith in him, and views of pardon through him: the Persic version adds, "and his sin is forgiven"; which, though not in the text, yet is a truth; for Peter's repentance was not like Cain's, nor Esau's, nor Judas's; it was not the repentance of one in despair, but was a repentance unto life and salvation, which needed not to be repented of.