Ephesians 4:17-18, Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds. They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart.
But Paul is writing to Gentile Christians in Ephesus. Why is he warning them not to walk as the Gentiles do if they are Gentiles? Because Gentile Christians are called to an utterly different kind of life than Gentile pagans. Pagans do not have godly understanding, separated from God because of their hardness of heart. Who are pagans? Any non-Christian—any who do not follow God in Jesus Christ. Who are the pagans in our day? Again, any non-Christian: Muslims, atheists, agnostics, Mormons, Buddhists, Hindus, etc. All these minds the Devil has darkened, all of whom are alienated from God because of their ignorance due to their hardness of heart.
In other places, Paul encourages all Christians to proclaim the Gospel to such pagans. Here, however, he issues an apostolic command: stop walking as the pagans do! That is, you do not believe, behave, or act as the pagans do. The pagan practices today include abortion, unrepentant homosexuality and transgenderism, premarital sex, promiscuity, divorce, pornography, and other things. Like the Gentiles in the Ephesian church, we are no longer pagans; we are now children of God and thus belong to God, who has called us out of such lawless ways.
No longer pagans, therefore, we are called to a holier way of living, restored in our relationship with God the Father through Jesus Christ in the Holy Spirit, called to love our neighbour as the Law shows us.