When it comes to being confident about something, there are many synonyms for the phrase "is sure of." Some of the most common alternatives include "knows for certain," "has no doubt," "is convinced of," "is confident in," "has faith in," and "is assured of." Other possible synonyms include "is positive of," "has trust in," "is resolved in," "has a firm belief," and "is steadfast in." Each of these phrases conveys a sense of strong conviction and belief in something, which can be useful when expressing certainty or confidence in your words or actions.