I don't know, I've been told that the Romans found the Jewish religion genuinely disturbing. Theirs was essentially animistic, after all, so each deity had a very real physical presence in their lives, but Yahweh was (broadly) characterised as an unknowable cosmic overlord, which they found rather intimidating. This may be apocryphal, but I've been told that when Pompey occupied Jerusalem, he barged his way into the Holy of Holies in the Temple, only to find that, to his horror, it was entirely empty; he had assumed that, as with European paganism, the inner sanctum would contain an icon of the relevant deity, so the lack of an icon suggested to him that the Jewish god must be some sort of undepictable monstrosity.
Also, the Romans apparently had a somewhat shaky grasp on the distinction between circumcision and castration, which sort of furthered the "insane cultist" interpretation.