I will always believe that John right there is thinking about when they first met and he’s feeling himself doubting that Mike didn’t tell Sherlock about him before they got there, which would mean everything else Sherlock had said on the roof was true, and John’s life would have been a lie since he met Sherlock. But he continues to believe in what he always believed in and as he says later ‘no one will ever convince me you told me a lie’, because Sherlock was and still is his best friend.