Why is Jesus' teaching about the regeneration not about salvation?
It concerns the coming regeneration of the earth, when it is made like the Garden of Eden and the Son of Man sits on His glorious throne. Salvation had already been established when Peter confessed, 'Thou art the Christ'; the question was about what would follow for those who had followed Him.
Les explains that Jesus is not talking about salvation because that was established back in chapter 16, when Peter said, 'Thou art the Christ.' Peter then asks what else they will have, much like Abraham in Genesis 15 asked, 'How do I know?' Jesus does not dress him down; He answers that in the regeneration—when the earth is brought back and made like it was in the Garden of Eden, the same thousand-year period—the Son of Man will sit on the throne of His glory. So the teaching concerns the future earthly restoration and His glory, not how a person is saved. During His earthly ministry, except the Mount of Transfiguration, He did not reveal His glory.
And Simon Peter answered and said, Thou art the Christ, the Son of the living God.