Be different? Stop doing what I used to do?
I wonder how many people in the first century got more than a little upset with Peter. They were probably mad at Paul and John as well, but here'they are thing "my 'bud' telling me to stop hanging out with my old crowd." Actually telling me "Enough already!"
What we have to remember is that he was talking to people that were living outside their original world. They were probably just like us – high school and college friends to party with. Frequent trips to the clubs or resorts. Maybe even some "Spring Break" antics.
But there it is, "they should be surprised that you are not running with them." He even makes it clear, that by being different, they (your old friends) are going to make fun of you. They will hurl names and insults at you for not being like you used to be.
His encouragement is that we "have had enought time of all that stuff," and we need to move on and be different.
How different? Different enough so that it is impossible for everyone not to see it.