In Ephesians 4:25, Paul transitions from general exhortation (put off the old, be renewed in mind and put on the new) to specific exhortation. We must put off falsehood and speak the truth with our fellow members of the body of Christ. The previous context in chapter 4 suggests that, more than the particular words we say, Paul is concerned with the worldview from which our words arise. He is admonishing us to abandon the language of falsehood and speak the language of truth.