His obligation was not to serve Greeks and barbarians, or the wise and the foolish as they saw fit, but as commanded by the Lord. Paul was sent by the Lord to preach the gospel to the nations. His obligation was to tell the truth and the truth often hurts. The word sin as Paul used it means a violation of the divine law in thought, word or act. In his epistle to the Romans, chapters 1-2, he shows that both gentile and Jew have violated the divine law.
Paul agrees with his jewish brethren that the way gentiles were living was heinous, and he gives the reason for their error. Romans 1:25, “For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen.”
In the United States of America, where I live, this happens all of the time. People typically do not make an idol out of wood or metal, and bow down to worship what they have created, but make an idol of persons. Entertainers and politicians are worshiped in the United States of America. The depraved passions and the depraved way of thinking that is seen in the U.S.A is a result of idolatry. This is not just among the unchurched but it is often among the churched.
A Christian is not one outwardly, who attends a church, but is one inwardly; he has repentance towards God and faith in Jesus Christ. The outcropping of belief will cause him to fellowship with other believers, to gather with other believers for preaching and teaching of the word. However, a Christian is not one who is seen outwardly but he is known inwardly by the Lord. A Christian is a bond-servant of Jesus Christ, obligated to preach the gospel in the world.