Final answer:
Paul the Apostle urges believers to no longer live as the Gentiles do in his epistle, Ephesians 4:17, promoting a lifestyle distinct from the prevailing cultural practices and aligned with Christian teachings.
Step-by-step explanation:
The question "Who insists that you must no longer live as the Gentiles do, in the futility of their thinking?" is addressed in the context of the Christian New Testament writings, where Paul the Apostle, in his epistles, particularly in Ephesians 4:17, urges believers in Christ to no longer live as the Gentiles do. This statement reflects the early Christian emphasis on a lifestyle that is distinct from the prevailing cultural practices of the time, which were seen as incompatible with Christian teachings. In the historical and religious dialogue, the insistence on ethical and philosophical transformation for those who adopt a new faith is common. This is exemplified by various religious teachers throughout history who have called for a change in behavior and thinking to align with religious principles, advocating a moral and spiritual life that stands in contrast to the commonly held beliefs or practices considered futile or misguided.