The phrase "be certain" can be replaced with a variety of synonyms depending on the context in which it is used. Some possible alternatives include "be sure," "be confident," "be convinced," "be definite," "be resolute," "be positive," "be convinced," "be assured," "be satisfied," "be confirmed," "be established," "be undeniable," and "be verified." Choosing the appropriate synonym can help to add variety and nuance to your writing, while also helping to convey the exact meaning that you intended. Ultimately, the words you choose should reflect the level of confidence and certainty you wish to convey to your readers.