Antonyms for the word "attesting to" are often words that convey doubt or disbelief. Some words that could be used as antonyms include: denying, contradicting, refuting, challenging, disputing, and contesting. These words all suggest that one is not in agreement with or supportive of a certain claim or statement. They indicate skepticism, and a lack of certainty in the validity of the claim being made. While "attesting to" implies agreement and confirmation, these antonyms convey suspicion and uncertainty. When trying to express a lack of agreement or doubt, consider using one of these antonyms instead of "attesting to".