The phrase "were sure of" can be replaced with a number of synonyms depending on context and tone. For example, the phrase "certain of" connotes a strong sense of conviction or confidence, while "confident in" suggests trust or reliance on something or someone. Other synonyms include "assured," "positive," "convinced," "secure in," or "guaranteed." Each of these words can convey slightly different nuances or levels of certainty. The use of synonyms can add variety and depth to writing and speaking, and can help to keep language interesting and engaging.