When you want to convey the idea of being certain or confident about something, there are several synonyms you can use for the phrase "be sure of." For example, you can say that you are certain, convinced, confident, or assured. Similarly, you can use "know for a fact," "be positive," "have no doubts," or "be confident about." Other synonyms include "have faith in," "rely on," or "trust." These words can help you express your level of certainty, whether it's about a decision you've made, something you've learned, or a belief you hold. Whatever synonym you choose, make sure it accurately reflects your level of confidence or doubt.