Romans 12:2 says: And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
(KJV).
The verse appears in Romans 12, the opening of Paul's practical exhortations after the doctrinal chapters. Read it in context at Romans 12:2, or the full chapter at Romans 12.