When Paul preached before Felix and Drusilla, he essentially had three points—righteousness, self-control, and the judgment to come (Acts 24). The fact that Felix and Drusilla were in an adulterous relationship did not prevent Paul from speaking very clearly about the justice of God. It was, if you like, almost a hallmark of his preaching. At the end of his address in Athens he says the same thing: “[God] has fixed a day on which he will judge the world” (Acts 17:31). The Bible makes it clear that we won’t escape detection or conviction or sentence forever. There is going to be a payday.
The idea that God is too kind ever to condemn sin and that everyone in the end will go to heaven does not actually find a basis in the Bible itself. Paul’s warning in Ephesians 5 is to those who have professed faith in Jesus, so that they will not pay attention to those who suggest other than what he’s teaching them, namely, that this day will come—a day that is fixed, a day that will be absolutely fair, and a day when the judgment rendered will be absolutely final.