Born again Christians who had the Holy Spirit can and do fall from grace. Our focus should always be on Christ and his power to keep all those that belong to him but apostasy is a very real danger to all Christians. Peter believed in eternal security until he denied his Lord three times. After repentance he wasn't so proud anymore.
You're wrong. Peter had not been baptized into the body of Christ by the Holy Spirit....the Comforter had not yet come when that happened. So your example is flawed. Those in the body of Christ are new creatures....having passed from death unto life. The only thing we await is the redemption of our body.
Romans 8:23
23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.