In his book ” A Higher Loyalty”, former FBI Director James Comey said that more than punishment, people change because of compassion.
That’s what happened to Peter, the rock.
Peter must have had experienced so much love from the risen Christ that with the inspiration of the Holy Spirit he wrote this great fact about love. He was changed guy after his betrayal to Jesus and realising that Jesus had long knew about it.
Do you want to change someone? Intensify the love, the change might happen faster than what have you realised.