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.
An 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 belief, allowing users to choose the best word for their specific context.
Dizionario di parole simili, diverso tenore, sinonimi, di invocazione per Idioma belief