The phrase "was sure" can be replaced with a variety of synonyms to add more depth and precision to a sentence. Some possible alternatives include "had no doubt," "was confident," "knew for certain," "was convinced," "was certain," "had faith," "was persuaded," "believed without question," "was resolute," "was emphatic," "had a conviction," and "had a strong feeling." Each of these synonyms emphasizes a slightly different aspect of certainty, from unwavering conviction to unshakable confidence. By using these synonyms, writers can paint a more vivid and nuanced picture of their characters or the situations they describe.