With this little word Peter is laying the connection between the coming words and his previous thought. He had said that Christians are done with sin and want to follow the will of God and now adds the reason why: for, because they have spent enough time doing Gentile sorts of things.
3 For the time that is past suffices for doing what the Gentiles want to do, living in sensuality, passions, drunkenness, orgies, drinking parties, and lawless idolatry.