(The following is taken from DAVID GUZIK COMMENTARY ON THE BIBLE, Studylight.org)
For we have spent enough of our past lifetime in doing the will of the Gentiles; when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries. In regard to these, they think it strange that you do not run with them in the same flood of dissipation, speaking evil of you. They will give an account to Him who is ready to judge the living and the dead. For this reason the gospel was preached also to those who are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
For we have spent enough of our past lifetime in doing the will of the Gentiles: Peter realizes we all spent enough time living like the world. Now we are called to live like Christians. It is a profound, and foolish, waste of time for Christians to live like the world, and we must simply stop being double-minded and start living as Christians.
They think it strange that you do not run with them in the same flood of dissipation: When the world looks at our godly living, they think it strange that we do not follow them in their flood of dissipation (wastefulness). If life lived after the flesh is anything, it is a waste.
Speaking evil of you: When we don’t participate in the sin around us, we convict those who practice their sin, and they don’t like that - so they speak evil of us.
“It does not matter how your good deeds are received by men. If you are like God, you will find them received with contempt and ingratitude.” (Meyer)