“Do not repay evil with evil or insult with insult. On the contrary, repay evil with blessing, because to this you were called so that you may inherit a blessing” (1 Peter 3:9).
Human nature is to treat others the way they treat us. If they treat us badly something in us wants to treat them badly. If they insult us our first instinct is to insult them in return. Peter is just reinforcing what Jesus said in His Sermon on the Mount. “But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also” (Matthew 5:39).
The reason Jesus gave is that in that way we are doing what God has done for us. He hasn’t treated us the way we treated Him because He is better than that. We ignored Him but He pursued us. When Jesus was cruelly mistreated He blessed those who abused Him. When we sinned against Him He died to save us.
Here, Peter says that repaying evil with blessing opens us up to receive God’s blessing. If we ask how we can possibly do this the answer is that we have a new nature that is a reflection of God’s own nature. When we are His we are given a new set of values and instead of acting like we once did we can act in a way that is consistent with the way He acts. If we bless others as He did we receive His blessing.
That’s not the way others, led by their human nature, react. It is one of the differences that mark us as God’s children. Acting toward our enemies as God reacted to us makes us eligible for the blessing of God Himself. That is only possible because He is changing us from who we were into the individuals He has created us to be.