The term "be dubious" implies that you are uncertain or skeptical about something. If you're looking for antonyms or words with opposite meanings, then "be certain" or "be confident" are good choices. Alternatively, you could use "believe" or "trust," which indicate a sense of faith or conviction. Other antonyms might include "acknowledge," "endorse," "embrace," or "support," all of which suggest a sense of trust or approval. Ultimately, the antonym for "be dubious" depends on what you want to communicate - whether you want to convey trust, confidence, faith, or conviction.