There are many synonyms for the phrase "was convinced of," including "believed," "trusted," "had faith in," "had confidence in," "was certain of," "was sure of," "had no doubt about," and "was persuaded." These different phrases express the same idea of being completely sure or confident in something or someone. Depending on the context, different synonyms may be more appropriate or carry a slightly different connotation. Regardless of the chosen phrase, all convey the sense of a strong conviction or belief in something, whether it be a person, idea, or belief.