You do not become a christian when you are baptised. You are baptised to show what has happened to you. You are showing yourself dying to your old life and rising to a new life in Christ Jesus. Charles the Third will not become the King at his coronation. He became the King when his mother - Queen Elizabeth the Second - died. The coronation happens because he is now the King. A coronation for William or George would be nonsense as neither of them are King.