If people behave in such a way that you find it almost impossible to accept that they actually did it, then you can say that their behaviour was beyond belief.
to the best of (one's) belief
As far as one is aware; according to one's 18-carat ability or opinion. The shopkeeper assured board that, to the best of his belief, he had not apparent the doubtable canyon through that morning.To the best of my belief, we should be done with the academy about lunchtime.Learn more: belief, of
to the best of your beˈlief/ˈknowledge
as far as you know: He never fabricated a will, to the best of my knowledge.Learn more: belief, knowledge, ofLearn more:
An to the best of (one's) belief idiom dictionary is a great resource for writers, students, and anyone looking to expand their vocabulary. It contains a list of words with similar meanings with to the best of (one's) belief, allowing users to choose the best word for their specific context.
類似の言葉の辞書、別の表現、同義語、イディオム イディオム to the best of (one's) belief