Antonyms for the phrase "was untruthful" can include: honest, truthful, sincere, reliable, credible, trustworthy, forthright, candid, and genuine. These words imply a sense of transparency and authenticity, conveying the idea that the person in question is truthful and trustworthy. While the phrase "was untruthful" implies a level of deception and dishonesty, using antonyms like these can help to convey a more positive or neutral connotation. When choosing which antonym to use, consider the context and tone of the message being conveyed, as well as the intended audience and their expectations.