They regard Christianity as polytheism and idolatry, due to the "holy trinity" and their worship for the Virgin Mary. This is an offence punishable by death under their Noahide laws. They also regard Jesus as an impostor and traitor to the Jewish cause, and Christianity generally as a menace to their existence, because it has vastly surpassed their numbers, and it has frequently served as a rallying point against Jewish supremacy. I would say, that started at the latest when we resisted the Muslim conquest of Europe at the battle of Toujours, when we started to push back and successfully reclaimed Spain and Portugal, which the Jews had given to the Moors before. And in the wake of the Reconquista, the Inquisition also helped to expel many Jews from Spain. I actually think it should have started much earlier, but I don't have tangible evidence. They also regard the white people, who are, or were, the main pillar that Christianity used to rest on, as their enemy, so hating on Whites and hating Christianity has been almost the same thing for a long time for them.